Put in freezer bag and place in freezer for at least 2 hours.
Add frozen banana slices to high speed blender and blend until smooth.
Either serve now (it will be like a soft scoop ice cream), or put in the freezer for 1 or 2 hours to firm up.
Either serve by itself or add a topping of your choice (strawberries, chopped nuts or maple syrup work well).
Notes
If your blender is struggling to blend down the banana slices you might need to add a little liquid to help it. Try using some type of milk, but only add a tiny bit at first so you don't risk making it too runny.