This thread is now about terraforming Mars AND Venus
Not necessarily both at the same time.
Mars. It's a planet, it's quite big (surface area nearly the same as Earth's land), and as it is now it is very hostile to human life. If we're going to talk terraforming, understand that it's a big job.
Mars has a negligible magnetic field. Earth's strong magnetic field protects us from some of the sun's radiation, but more importantly (in the long term) our magnetic field protects us from having our atmosphere stripped away by solar wind. Mars once had an atmosphere, despite its smaller mass, and some theories have it that Mars had a magnetic field to protect that atmosphere. I tend more towards the theory that the Martian magnetic field never was enough and it just ran out of gas.
Giving Mars a magnetic field and waiting for it to capture its own atmosphere, doesn't seem like a practical course to me. But a magnetic field would be fantastic, if you can think of a way. The next section deals with ways of providing an atmosphere, and atmosphere itself protects against solar radiation. The added protection of a magnetic field would be bonanza.
Okay, atmosphere. I propose to intercept comets and crash them into Mars. Comets are mostly water.
An atmosphere of water vapour would be a great start. Wait you say, Mars is far too cold for that. Well actually. If you put enough ice on the surface it would sublimate, creating an atmosphere which captures and retains heat (water vapor is a greenhouse gas) which would melt the ice around the equator and evaporate some of it. With feedback like that, we'd have an atmosphere of water vapour in no time. Say a few decades.
Then solar radiation beats up on the water molecules, creating hydrogen and oxygen. Hydrogen is far more mobile and escapes the planet's gravity more easily than oxygen does (though even oxygen escapes more easily than H2O) and we have an unbreathable atmosphere with some oxygen and some oxides.
We have nowhere near made Mars habitable for humans, though. Mars is covered in reactive minerals. The available oxygen is going to get used up, for centuries, no matter how many comets we crash into it. In fact, we can't go too far with adding water that way or we'll make a deep atmosphere with clouds (like Venus has) and solar degradation won't do much to it.
Ideally we would add pure oxygen but I don't see where we would get that. Oxygen on earth is a product of life, it's scarce to us. While extracting oxygen (eg from water in Mars orbit) would take too much energy.
Is the terraforming of Mars a century out of our grasp? Or is there some clever way I haven't thought of?