Put a piece of the peel into a clean glass jar and add a tablespoon of wholemeal bakers flour and a tablespoon of tepid water, and mix together. Cover with a muslin cloth or open weave tea towel.
Day 2: Uncover jar and add another tablespoon of water and of flour and mix well with the rest. Cover and leave in a warm place.
Day 3: You should notice some bubbles beginning to form and a pleasant yeasty smell, and possibly a slightly acidic smell. Add another tablespoon of flour and of water and stir.
Day 4: Fermentation should be happening by now. Add another tablespoon of flour and tepid water and mix, and remove the potato peel.
Day 5: Your leaven should almost be ready. It should be nice and bubbly and smell sweet and tangy. Add another tablespoon of water and flour and stir.
Day 6:You should have a proper leaven by now. Leave it in a warm place and then weight it. Add equal amounts of water and flour to match the weight of the leaven. ( If you have 150 gms of leaven , add 75 gms each of water and flour. This is called 100% hydration.)
This leaven came straight out of the fridge. It will develop more bubbles as it warms up.
Use whatever you need for baking, keep some and discard the rest. If you keep it in the fridge you will need to feed it once a week, as above.