Stage 1. Mix 500g of the flour with the apple and water. Tip this into an airtight container and mark the level on the outside of the container (so you can see whether the mix has risen). Cover and leave to ferment for 3 days.
Stage 2. After 3 days the mix should start to smell quite sweet, a bit like cider. It will be a little darker in colour and will have started to grow; it may also have some bubbles. Check the level against the mark you made on the outside to see how much it has grown. Discard half the mix and add another 250g bread flour and 170ml water – this is called ‘feeding’. Mix thoroughly in the bowl. Tip back into the container and leave for a further 2 days.
Stage 3. There should now be plenty of activity in the dough, indicated by lots of small bubbles. If there is nothing happening, look at the side of the container – you’ll be able to see whether the dough has risen and fallen by the smearing on the side. If it has risen and fallen, then it is active. If your starter is active but has sunk down in the tub and a layer of liquid has formed on top, then it is actually over-active. Stir in some more flour to return it to a thick consistency and leave for a day. It should regain the thick, bubbly texture you want. If there is no sign of rising on the container, and no bubbles, leave the dough for a couple more days.
Once your starter is active, discard half of it, as before, and mix in another 250g bread flour and enough water to return it to the consistency of a very wet, sloppy dough. This time leave it for 24 hours. If the starter begins to bubble within this time, then it is ready to use. Ideally, when you come to use it, you want to starter to be thick and bubbly. If you shake it, it should wobble like a jelly, without dropping down. When you put a spoon through it, it should be like a thick batter. If your starter is not bubbling, feed it again, following stage two, and leave it for a further 2 days.
If you are using your starter often, you can leave it at room temperature, feeding it at least every 3 days and whenever you take some to make bread. Simply stir in some strong white bread flour and enough water to return it to the consistency of a very wet dough, bearing in mind that you will need 500g starter for each recipe. Then leave it, covered, until it achieves that thick, bubbly, jelly-like stage. If you are making sourdough less often – say, once a month – then keep the starter, covered, in the fridge. This will slow down the activity and preserve it almost indefinitely, but you must let it come back to room temperature before use. If it seems to be inactive, give it a feed of fresh flour – the bacteria within it are living so they need feeding.