When you get in there and look (if you haven't already) you'll see players are grouped into three categories: A, B, C.
You pick two from A, four from B and two from C for each tournament.
Once you select those four guys you are stuck with them for that tournament (but can change them up the following week).
You play one from A, two from B and one from C for each round.
You can only use a player 10 times throughout the season so you have to choose wisely because you cannot simply pick Mickelson, Woods, Singh and other top performers for every week. It's like hockey in that sense, once the magic number of games is used up, they're not even in existence for you anymore.
Beware whether or not guys are actually playing. If you pick players on Sunday prior to the tournament you may find they aren't even in the tournament. Best to check on Wednesday night to make sure your guys are eligible. I've had guys drop out for odd reasons last year at the last minute too - illness, family problems, drug problems
LMK if you have other questions. max_one get in there too please, the more the merrier.