I don't stew apples for crumble, just chop them, sprinkle a little bit of sugar over the top, then the crumble mixture (butter, flour, sugar, hazelnut meal/almond meal/coconut/oats/macadamias dependent on cupboard status)

But if you want to stew, I did it this morning, and was deliciosa. Chop your apples to desired size. If you want to go mushy, chop them quite small and use a dissolvey type apple like Golden Delish, or puree after stewing. Toss them all in your saucepan with about 1tsp water for each apple. I like to add cinnamon. If using a sour-ish apple (this morning I used home-grown granny smiths) I add a bit of raw sugar.
Low heat, lid on, give it a shake every now and then till they're soft - about 10 mins, but keep an eye on it.

Good luck!