how do you get steam cards from games exactly?

You do not have to play them, as long as they're open you have the chance to get them.

You don't have to play them (who plays games anyway? You just leave 'em idle), leaving the game open is enough

start the game and after go to youtube!! :)

Fwiw, I've found most games require about 90 minutes to cough up all their cards. I've had a few be much shorter (45 minutes or so) but they're the exception.