Love crumble! It’s so easy to make. The big, big rule is never to add the sugar to the crumble until the butter is rubbed into the flour. The reason is that the sugar and butter will cream; that means mix together, a bit like a sponge mixture and the crumble will be heavy and more like pastry. So gently mix the sugar through with a spoon, at the end.
Then just pour over the cooked apple, do not pack down, leave as light and fluffy as possible.