You choose the people in your party, then the date, then the park. It will then show you a list of attractions with a few times available next to it. There will be more times available than that, click "view details" and it will give you more times to choose from.
Like PP said, go for the hardest ones first. When I did my FPs, I did my 4th day first, because that was our first AK day, and I wanted FoP. Still couldn't get it. Went to day 6 and was able to get one. I didn't want Frozen, so no problem there. I was able to get 7DMT several days after our FP window opened. SO, for us, only FoP was a problem.