First thing to do is to navegate to your game, and right click on the text, and go to properties. From here you will see a "Set Launch Options" box, and click on that. In there, paste or type this line of code:

-enableconsole

And that's it. Your game should have the console enabled. Now, on to how to use it.

Whist in-game (some require you to be playing, that's a given) press the Tilde key (`) and you'll see a small green box appear at the bottom of your screen asking to enter text. Press it again to make this bigger or not.

Using the console has it's advantages, such as disabling HUD for better screenshots, or if you want to be a loser and and enable god mode to complete the game.