
As above - 6-3-3 formation! Whatever weight you use of flour/butter/sugar, use 6 parts to 3 to 3. I've never been able to make crumble at all til I found that out from youtube and I can do it perfectly every time now!
Other tips - use Trex veg lard, friend of mine is a superb cook and used to do 200 puddings a day at a school and she swears by it. As far as I'm concerned her word is gospel when it comes to food :-) And, she also says to use only demerera sugar. I've dutifully followed these things - and I've used plain flour, not self raising but I don't think it matters (as long as you allow room in the tray for the raising, I did a bad batch years ago with SR and ended up with a nearly permanent ring of not-crumble on the bottom of my oven :-P) - and got lovely crumbles with raspberries, rhubarb, and rhubarb & raisin in the freezer waiting for winter :-)