The first thing you'll need is an image editor that can handle layers.
I'm going to be demonstrating this in GIMP just because (a) I have it (b) it's free (c) it works on most platforms. If you run Windows all this should work just as well on Paint.NET and if you have the cash to shell out for Photoshop or Illustrator or whatever... you lucky bastard. >.<
1: Start big.
The base version of your map should be large. Larger than it needs to be, even. You can shrink it down later if you want and it'll survive with minor adjustments. Trying to blow up a small map won't be nearly as effective.
2: Work out a scale. It's not strictly necessary to do this first, since you can work it out later from the overall dimensions of your map, but it's always a good idea. This makes it easy to calculate distances (with the measuring tool or its equivalent in whatever software you're using) and is generally a helpful guide to how far apart cities, climatic zones, national waters, etc should extend. For a map of this size, I'm keeping things simple: 1 pixel per kilometer (a scale of about 1:2,834,628).
What it looks like now
3: Draw your borders. Even if you're the kind of person *coughpedantcough* who doesn't approve of any maps not drawn from the plate tectonics upwards, you're going to need at least a vague outline of what your country looks like in order to determine where exactly said plates are. Borders should be on a new layer, since you'll either want to duplicate them often or be able to select the inside of their area for colouring in. Draw at around 400-500% zoom, even if you're very sure of yourself; it's much harder to get details in at 100%, particularly for those unfortunate souls using trackpads or clit mice like myself.
This brings us to the first practical consideration. Realistic-looking borders. Geography is fractal: highly detailed at every level of zoom. Your borders should not be blocky or blobby -- not rounded, certainly not square. They should be jagged. Coastlines have bays, coves, capes, estuaries, peninsulas, and straits. Borders may follow mountains or rivers (although they may also be straight lines, typically when set by treaty); random curves and whatnot do not look particularly realistic. Let's say you're drawing an island country with an inland sea. It might look a bit like this:
Borders at 67% zoom
This also brings us to the first stylistic consideration: to antialias or not to antialias?
I used the paintbrush tool solely because it does antialias -- the lines look neater that way in my opinion. However, it's also inherently less accurate, due to the nature of antialiasing (it introduces additional pixels at varying opacities to smooth the lines), and some people prefer the non-antialiased look. Here's a side-by-side of my antialiased drawing with Non-Antialiased Island for comparison purposes.
The next steps will focus on some common overlays. With layers, you can create a number of different maps (physical, political, urban areas, topographic, bathymetric, etc) covering the same area.
4: Rivers, Oceans & Lakes.
Colouring in oceans is easy (I'll deal with bathymetry in a separate section). Select the outside of the borders with the magic wand tool, create a new layer, select a suitable shade of blue and click anywhere inside the selected area with the paint bucket.
What this looks like
Rivers and lakes are slightly more time-consuming. First, invert your selection (ctrl-i in most programs) so that the area inside the borders -- empty on this layer -- is selected. Second, brandish your paintbrush or pencil tool and get to work. You can draw rivers in the same shade of blue as the oceans, or make them darker or lighter; some shade of blue, however, is customary.
What this looks like
There are a number of important things to remember about rivers. Typically, rivers start at high elevations and flow towards low elevations, according to gravity. As such, all the rivers on this map start inland, in areas where elevation is likely to be high, and flow to the sea. River basins may also be endorheic (i.e. with no outlet to the sea), typically in desert regions or other areas where the river flows below sea level. Freshwater rivers do not flow from one sea to another. The only "rivers" that do this (like New York's East River) are actually tidal straits. Likewise, although this may seem incredibly basic, rivers "rise" somewhere (water comes out of the ground) and have a "mouth" elsewhere (where the water flows into the sea/a lake/a delta/a salt pan/another river/etc). You will not see circular rivers or any such silliness.
Rivers often have tributaries -- smaller rivers that flow into them. I've shown a few. Distributaries, where a smaller river branches out from a large one, are much more rare -- I've only included one. The only time you'll see lots of distributaries is in river deltas, where the flow rate decreases dramatically and silt carried by the river forms natural dams that cause numerous bifurcations. If you make a map which has lots of rivers that split into smaller rivers flowing different directions, you should be aware that this is rather unrealistic -- particularly if it occurs at high elevations. Rivers follow the path of least resistance. That's path, singular.
Meandering or braided rivers are common in the lower reaches of any stream's course; I've shown a couple of examples of what these might look like on a map. Rivers may flow into a delta or an estuary instead of simply falling into the sea direct; there are examples of those, too. Rivers typically don't just stop in the middle of nowhere, but as mentioned this can happen in endorheic basins -- if your nation has a desert climate or highly karstic (limestone) terrain, rivers can drain to inland deltas or depressions, disappear into sinkholes or drain to isolated lakes whose only outlets are seepage and evaporation. Rivers also tend to have great erosive power, and will carve out valleys that are lower than the surrounding terrain.
As for lakes: they can be fed or drained by rivers, but that's not compulsory. Lakes with no inlets or outlets are quite possible, and lakes may really be of any size. The only hard and fast rule is that however many inlets a lake has, there will generally be only one (natural) outlet. Path of least resistance, as above.
5: Topography & Bathymetry.
Topography is better known as "height on land"; bathymetry as "depth at sea". The most common way to express both is with bands of different colours. On land, one can include anywhere from three (green for low, yellow for mid and orange, red or white for high elevations) to dozens of colours. Here's an example from a website that makes maps professionally. I'm going to use six, a colourscheme I've seen a number of people use although I'm not completely sure who originated it -- Belkaland, perhaps? -- incidentally, anything you do steal should be credited <.<
Still have the interior of your borders selected? Good. Make a new layer, hit the paint bucket, use the shade of sea level green and colour in the whole thing. If this hides your borders, you can drag the layer with your borders on it up to the top of the layer stack. Also drag your new layer below the rivers/oceans/lakes one, so that water features are visible. This is important because you'll probably wind up drawing a fair bit of terrain around them.
What this looks like -- note layer stack organisation
This is likely the most time-consuming part of drawing maps: the contours. There aren't any good shortcuts I can think of for making these. Just select the next colour, switch to the pencil tool, and start drawing. It can be a good idea to sketch the outlines of mountain ranges, river valleys and depressions in a sketch layer if you want. (Note that at this point I opted to delete the ocean, since it was hiding the colours key -- I can always restore it later.) Also remember that the same principles that apply to coastlines apply to contour lines -- they're very uneven. Eventually, you should have something that looks a bit like this.
What this looks like. You can see what I meant by "drawing around water features".
Obviously, there are different ways -- all of them time-consuming, but some you may find preferable. Instead of different colours one can model terrain with the contour lines familiar to anyone who's looked at maps by the USGS/OS/Atlas & Gazetteer/your country's equivalent. This will take just as long and won't look as pretty, but there are advantages: it's easy to find the specific elevation of any one location, one can add colour overlays indicating tree cover or parkland or private property or whatever, and rivers can be easier to see (blue on green doesn't always come out well). Or one can simply skip the topo maps altogether and rely on terrain maps to get the message across. I'll cover that in a later section.
Ocean depths work the same way. Find a few colours (I'll use four) and draw continental shelves, trenches and whatever else takes your fancy. Just remember to invert your selection so you're colouring in the outside of the borders. :P
What this looks like. I've also copied the colours key down to the bottom for our legend, although I don't think you can see it in this screenshot.
6: Terrain & Land Use Maps
But topography, you might argue, is unnecessary. The precise elevations of everything in my nation, though they may add verisimilitude, are not that important to me. Moreover, it's a lot of wasted effort because it requires a lot of work and doesn't include lots of information I need to know: how much arable land there is, what natural resources my country has, where the deserts are....
If something like that was your first thought upon reading the section above, this may be the place for you. And if you survived topographic maps, land use and terrain maps are much easier to make. This is, however, an area much prone to abuse, largely due to simple ignorance. I've seen people plop down deserts right next to tropical rainforests. Obviously it's governed by the Rule of Cool to some degree, aka "But this is NS!" -- people think it's cool to have a desert, a jungle, glaciers and tundra all over the place, with volcanoes sprinkled wherever they're convenient.
It's best to do your research before starting. Determine your approximate latitude, and figure out how the ocean currents will affect your climate. I'm drawing an island country, and I'll decide to stick it just north of the Tropic of Cancer with prevailing currents from the southwest. The climate will be humid subtropical in the south and west, drier around the inland sea and more oceanic in the east. There'll be a desert in the south central region of the country due to the rain shadows from nearby mountain ranges.
The first step of a terrain map is to make your key. Create a new layer and add something to the legend looking a bit like this -- colours and symbols I'm selecting pretty randomly. One of them's stolen from a NS trophy; you could use those, or just existing geometrical brushes in different colours if you don't feel like defining new brushes as I did.
Now, do what you did with topography: start filling in terrain, define some new brushes and plop down resources.
The coloured-in map raises another question, however: placement of cities. (Cities are surrounded by built-up areas, which are brown on this map.) I'll cover it in detail in the next section, but cities will come up in RP quite a lot and shouldn't simply be placed at random. Don't worry about them for the moment.
The symbols don't come out well at 67% zoom but here's what a portion of the map might look like at 100%:
7: Cities
Cities should be a separate layer. You can make a wide variety of political maps involving them, and you won't necessarily want terrain or topography interfering.
If you're being realistic, however, cities don't come into being at random. They are born at crossroads, at river junctions, at natural harbors, at oases and at places rich in natural resources. They might come to occupy positions of strategic importance, or they might simply be the one place where the land is flat enough to farm and the river narrow enough to cross. Though a few cities will be purpose-built to serve as capitals, labour camps, or any number of other things, above all the locations of cities are determined by the nation's geography.
Switching back to my terrain view, I'll take a moment to identify a few likely sites where cities might come into being. Illustrating the process consciously looks something like this, although I'll obviously take many more considerations into account. Not excluding "parts of the map that would look good with more cities" either. <.< Cities are going to be much less common in deserts, icy tundra, rainforests, high mountains and other such extreme terrain; they'll cluster around river valleys and along coasts, and spread out through agricultural land.
Once I've finished adding cities, I find that lo and behold, they map perfectly onto the built-up areas I drew into my terrain map. What a strange coincidence. You probably won't be so lucky, so draw the rest of the terrain first, then the cities, then add the urban areas.
The cities are just dots -- the larger the dot, the larger the city -- due to laziness on my part. There are other solutions; here's a common one.
Now that you've got geography and cities, you have all the information necessary to draw up internal administrative divisions. Divisions are much like national borders: they will follow mountain ranges, rivers, and other features; and may also be straight lines arbitrarily set as such, or compromises between disputed territories. I don't feel like drawing any more right now though and I think you guys get the general idea by this point.
Questions, criticisms, anything I overlooked or was off base with, all welcome. If there's anything else you want me to demonstrate let me know.
