Freezing bananas is a great idea for several reasons.
Choose ripe bananas.
Peel your bananas before freezing. You can freeze with the skin on but then you'll need to thaw them out and peel before using them. This makes for a very unpleasant looking, slimy banana. Fine for banana bread but not too appetizing for anything else.
Slice your bananas as desired and lay out on a freezer paper lined cookie sheet. I like 1 to 2 inch pieces. Bananas can also be frozen whole.
Place in the freezer. When frozen solid package in freezer bags. This freezes each piece separately and allows you to remove just as much as you need.
When you package the bananas remove as much air as possible from the bag. Be sure and date and label your freezer bags.