Slice the bananas into 1/4 inch chunks and lay them out on a tray or plate that will fit in your freezer.
Freeze your banana chunks for a couple of hours until firm.
Blitz the frozen banana in a blender or liquidiser until you have something the texture of baby food.
If you like you can swirl through other flavourings at this stage. Some people eat the ice-cream like this but I prefer to put the slushy banana mixture into a tub and re-freeze for at least another hour to get a real 'ice-cream' texture