Peel the apples and put the peelings in a sterile glass with the flour and the sugar.
Add the water and put on a loose-fitting lid – the dough needs to breathe.
Leave it in room temperature for 4-6 days until it starts to bubble and froth.
It is important that the sourdough develops in a draft free room. The sourdough keeps for a week in the fridge without needing to be fed.