A dead blow hammer works better than a mallet, there is no (or less) bounce so you don't get a small shadow impression as well. With a dead blow hammer, if you didn't hit hard enough the first time, you can usually reposition the stamp by feeling where the stamp settles into the impression, then hit it again. If one side is light, you are tilting your stamp. So tilt it the other way and hit it again. Warning, on thin leather, you can cut right through the leather if you hit it too hard or too may times. Do some trial runs on scrap so you can see what it does.
Also do a search for arbor presses here. There are lots of threads and good info.
Tom