hear in Canada you fill out a form and send it to the government it is done as part of a gun sale if you owned the gun pre registry you can register it talk to the friendly staff at your local gun store they can walk you trough it they do it everyday
as a side note historically gun registration has ALWAYS lead to confiscation most recently in Australia and the UK and alarmingly right here in Canada mostly handguns that they suddenly deemed illegal so if you are lucky enough to own a unregistered gun leave in that way un registered because it and you is safest that way but in the end the choice is yours
Additional answer: it depends on where you are. In the U.S. there is no mandatory registration nationally for regular firearms. A few states require it, but only a few.