Boil a jar to sterilize.
Grate the apple (with the peel) into the jar, then add the water and sugar. The jar should be filled about 70% full.
If you have any, add fermented liquid from raisin yeast or any other yeast to speed up the fermentation. (This step is optional.)
It is ready when the lid makes a popping sound when you open the jar, and you hear fizzing. The grated apple will pop out of the jar from the fermentation.
These bagels (https://cookpad.com/en/recipes/147564-bagels-made-with-apple-bread-starter) use the yeast shown here.