Each FastPass attraction has about 8-10 FastPass machines. You insert your park admission ticket, and get back your park ticket and a FastPass ticket. The FastPass ticket displays your return window. Come back at any point in the range indicated, and make sure you get into the FastPass return line. Some of them are not clearly marked, so ask a Cast Member if you're unsure.
Eventually you'll be asked to present your FastPass ticket, and the FP queue will deposit you near the front of the line. Don't always expect to be the very next rider, particularly on attractions that have pre-shows. But you can rest assured that the wait is MUCH shorter than going through the standby line.