How to register undoented pistol?

I bought a pistol from a friend in Arizona. The laws there are pretty lenient when it comes to firearms. I now live in Michigan. Here to get a firearm registered you have to have the FFL dealer fill out the paper work you get from the police department, then you take that paperwork back to the police and they finish the registration. How would I go about registering my firearm if I have no paper work on the purchase?