This is so easy and so well known, I hesitated to post it but it's my blog and I'll do what I want, to coin a phrase, so here goes. :-)
It's a three ingredient mixture with optional extras.
For each portion of plain flour, you want about half that amount of butter and less than half that amount of sugar (the sweetness is to taste really).
So if you have 100g flour, you will need 50g butter and between 30 and 50g sugar.
(Recipes vary in their amounts so it obviously doesn't really matter all that much)
Put the flour in a bowl and add the butter cut into cubes. Rub in the butter until it resembles breadcrumbs.
Mix in the sugar.
And that's it!
If you want to freeze some, just pop it into a plastic box or a poly bag and when you next need some, just spoon out what's wanted. Dead easy!
There are loads of variations. My mum used to add chopped walnuts and oats which was lovely. I've seen recipes with desiccated coconut added to the basic mix and sweet spices usually go well with the fruity bottom. Or you can use different sugars - demerara gives a nice crunch, for example, as well as a deeper flavour. One other addition that really appeals to me is to use grated lemon zest.
Frugal tip: if you use caster sugar (you don't have to for crumble topping), pop some granulated sugar into a zizz and grind it down to the right texture. With granulated sugar currently £1.00 for 2k in Morrisons and caster sugar at £1.60 for 1k, it's a no brainer!
(However, only zizz down what you need each time)
You can also continue zizzing it down into icing sugar, if that's what you need.
As for the fruity bottom, all I will add is that rhubarb and strawberry is a flavour marriage made in heaven and I have both in my freezer, produce from last summer's garden and allotment.
Do I need to say more|?