I have one of those hammers with rubber on one side and hard yellow plastic on the other. I also have a small jewelers hammer but I don't use it as it is too light. If you use a standard metal hammer, be really careful not to hit too hard, or in a glancing blow kind of way. The metal is fairly soft and will mar easily. The technique Bill layed out is how I do it, but I am not good enough to get them only with 5 hits. After the initial 5 hits, I usually need to tap the ends down again and then tack them in with water thin superglue.
The biggest mistake you can make is letting the fret 'roll' to one side when you hit it. This will almost always take a chip out of the fingerboard on the slot edge. Believe me, it is very easy to have happen too! A lot of builders will actually bevel the edges of the slot a tiny bit to help keep this from happening. If you have a piece of practice fingerboard, (stewmac includes a small piece for you to practice on) I suggest you use it to get the feel of how they go in. Once you do a couple, it is pretty easy.