How do you make your own drum beat completely from scratch

I want to make a drum beat that keeps time while I'm practicing my swing dance. So in swing dance we have 6 count and 8 count steps. So I want to just have two eight counts which accent the last beat so that I know it's over, then I want to switch to two 6 counts, accenting the last beat. It looks like I can't do that with Garage Band. With garage band you can just make small variations on existing drum tracks but you can't write your own track completely from scratch.