Here's the short answer, although those links will probably help more.
A couple of days before you brew, pour off most of the beer on top of the yeast, then swirl the rest up so it's all mixed together. Add that to a liter or so of fresh wort. You can cover it loosely (it needs some air) and shake it or swirl it when you think of it. (Eventually, you'll want a stir plate and flask, but for now, a mason jar covered with foil will work.)
Make your beer and chill it to pitching temperature. Add the yeast slurry (the starter you just made) and ferment as normal.
You can use the calculators above to figure out how much you need, because you probably have more than enough for a batch. But you can also just add a cup of the slurry to your next batch and see what happens!
Sanitation is important when you collect the yeast, make your new starter, pitch yeast, etc. Sanitation!