There are a few things you can do to make sure you get perfect topping, every time.
Hand mix vs food processor - You can either mix the ingredients for the topping together by hand or use a processor. Personally, I like to do it by hand, as you have more control over how fine or thick you want the crumbs. It's also less washing up which is always a bonus. If you do use a food processor then pulse it on and off so you can keep a close eye on the texture of the crumble.
Butter - use slightly softened butter that has a little give to it, but is still solid. If you use butter that is too hard then it will take a lot longer to incorporate.
Oatmeal - oatmeal or rolled oats are a popular addition to the crumble topping. If you want to add oatmeal then the simplest way is to sprinkle a few tablespoons over the top so they get nice and crispy in the oven. Alternatively, you can substitute 50g of the flour in the recipe below with oatmeal if you would prefer them worked into the topping.