Take a Banana, two of them, NOT bright yellow, it is better if you use those somewhat black. Now, smash them, LIKE A HULK SMASH PONIES! Smash smash!
It is easier with electricity, but if you do not have it- take a fork and SMASH IT with a fork!!
Add milk, and sugar( just so that milk cover the HALF of smashed "a BANANA" and mix it together. IF you want it cold, now is time to add ICE( frosen water), anyway, add ice if you want.
Then add rest of the milk, more if you want it to be more liquidy, less if you want it to be more Bananany(tm). Add sugar, optional cinnamon, optional alcohol.