I use a piece of scrap steel clamped in place as a guide, with masking tape on the steel to mark the proper spacing. Hit it hard and hit it ONCE. Chances are if you try again you will get a double strike. It is very difficult to line up properly to hit it again and have it look good.
I just put my completed receiver in a heavy vise or on a concrete floor to do the stamping. Make sure your setup is stable and hit the stamp square.