I started making it homemade Apple Cider Vinegar when I was looking for things to use up the wind fall from my apple tree's.
You make it using all the apple cores, peelings and bad bits you don't use.
Put all the pieces of apple in a bowl and allow to brown over about 24hrs.
Pack out a large jar/container with the browned apple pieces and pour in enough water to just cover the apples.
There are loads of slightly different recipes on the Internet but this is how I've been making mine. Cover the jar with an old muslin/tea towel and put somewhere warmish and dark and leave for about 2-3 months. I leave mine in a cupboard in the kitchen which may be why it takes so long. Lots of people leave it in the airing cupboard (warm and dark - I'm going got try the airing cupboard next time and see how it changes the process).
You can keep adding apple peelings etc to the mix (make sure you top up with water) until you start to get a white grey scum forming over the top.