If you are staying at a Disney Resort Hotel you can book ALL the FP for your trip 60 days before you arrive. That is - 3 per day (for one park per day) for the duration of your 7 day ticket.
If you're not staying at a Disney Resort Hotel you can book 3 FP for the first day of your trip 30 days before you arrive. You must then visit the website again the following day to book 3 more FP for the second day of your trip, and so on, up to the 7 days on your ticket.
Once in the parks, when you've used your 3 FP you can attend one of the park kiosks and book another FP, use it and return again. This process is repeated as long as the availability of FP remains.
If you're Park Hopping you can only pre book FP 30 or 60 days before you arrive in one park per day. If you visit MK and use your 3 pre booked FP you have to Hop to your next park and book FP at a kiosk in that second park. There's no booking EPCOT FP in MK (for example). Or you could go to MK first and have your 3 pre booked FP waiting for you when you get to EPCOT - just remember that you can't get any FP in MK as you have 3 already booked for later that day.
Easy isn't it !!!!