There are several different ways to crack your own back.
However the way I crack my own back is to stand up and then place both my hands behind my back with one hand on top of the other.
Place your hands at the center of your spine with them being on top of one another.
Next press both of your hands against your spine and then as you're doing that then lean backwards.
Continue leaning further and further back and you should hear and feel a slight popping sound which means you've successfully popped your back.
If you feel any pain or other discomfort while popping your back this way then stop because you could cause an injury to your back which could possibly leave you paralyzed or cause more injury and make your back hurt.
A good chiropractor can also pop your back as well much more safely if you're not sure how to do so yourself.