Heat the ingredients gently over the medium low heat and mash the berries as they break down, you could use an immersion blender if you'd like.
You could also microwave the whole bit and mash at the end.
I like to make a larger batch like this to store and have on hand in the fridge for busy school mornings. There is no magical ratio here, you can tweak any of the ingredients to suit your (or your kids) tastes!
Alternately, for a smoother texture, you could puree everything in your high powered blender