The app is My Disney Experience. Download and use it! Book your first 3 ahead of time at the 30 or 60 day mark. You can book on the MDE website or using the app. As soon as you scan your Magicband or card to enter the ride for your third selection use the app to book your next. You can literally book your next one while in the FP+ line. You will be choosing from what's left over so my advice is to go at rope drop to get in a few rides before the crowds grow and then have your 3 FP+ selections right after, still fairly early, and then book your 4th, 5th and so on. Just get those first 3 out of the way as soon as possible so you have a better selection of your next FP+. Headliners like Frozen Ever After and 7DMT probably won't be available same day so try to get those ahead of time.