Most of the popular attractions have machines situated on one side or the other that distribute Fast Passes. There is a clock above showing what the return time is so you can decide if you want a FP for that attraction or not. Of course, if the stand-by line is short there is no reason for a FP at that time. You enter your park admission ticket into the FP machine and it will print out a FP that will name the ride/attraction (don't forget to take your park admission ticket out of the machine), the time window when you can return, and the next time another FP is available. When you return you just show your FP to the Cast Member at the FP entrance. Another CM will collect your Fast Passes as you get closer to enter the ride/attraction.