Start with a ripe banana-it should be sweet and soft. Peel the banana and cut it into coins. It doesn't matter what shape or size the pieces are in as long as they are chopped up into evenly-sized and somewhat small pieces. Put the banana pieces in an airtight container. Freeze the banana pieces for at least 2 hours but ideally overnight.
Blend the frozen banana pieces in a small food processor or powerful blender. Pulse the frozen banana pieces. A small food processor or chopper works best. Keep blending-the banana will look crumbly.
Scrape down the food processor. Keep blending-the banana will look gooey. Scrape down the food processor. Keep blending-the banana will look like oatmeal: It will get smoother but still have chunks of banana in it. Scrape down the food processor.
Suddenly, as the last bits of banana smooth out, you'll see the mixture shift from blended banana to creamy, soft-serve ice cream texture. Blend for a few more seconds to aerate the ice cream. (If adding any mix-ins, this is the moment to do it).
Transfer to an airtight container and freeze until solid. You can eat the ice cream immediately, but it will be quite soft.