how do i put the rear brakedrums back together on my chevy tracker

Did you install new brake shoes or just pull the drum off? You may need to adjust the brake shoes in a little so the drum will fit over them. Here is a link for a generic brake drum illustration --- http://auto.howstuffworks.com/auto-parts/brakes/brake-types/drum- brake5.htm

If there is a lip on the outside edge of the brake drum due to wear, I would have the drums turned at a brake shop.

If you want to change brakedrums then it needs good knowledge of mechanism, if you doesn't have that much knowledge then don't dare to change it on your ownself. It can harm you vehicle.