Step by step recipe:

1. 2 min.
   Preheat the oven to 210°F (100°C).
   
   Rinse and dry 1 apple.
   
   Remove the core with an apple-corer (as in this photo). You can peel the apple, if you prefer, but this is not essential.
2. 3 min.
   Slice the apple(s) very thinly. The best tool for this is a mandolin.
3. 2 min.
   Arrange the apple slices, without overlapping, on a sheet of cooking parchment and slide this onto a baking sheet.
4. 1 min.
   You can sprinkle a little caster sugar over the apples if you like, but this is not essential.
5. 2 hours
   Bake in the oven for about 2 hours.
   
   To check if the crisps are ready, take one off the baking sheet and leaving it to cool. It should be brittle and crisp, not soft and bendy.
6. 
   Unstick the apple slices from the paper (which you can reuse) while they are still hot and leave to cool on a wire rack.