Initially, I used the hammering method, but I find pressing the frets into the slots far more expedient, faster and easier.
All I'm using is my cheap drill press and a little gadget I made, which is simply a rectangular piece of wood whose length is roughly the same as the longest fret I'm likely to use, in which I inserted a small length of dowel. It has to be hardwood to be effective and to last: I started with a piece of mahogany -- far too soft --, then glued on it a piece of maple -- still not hard enough -- and finally glued on a scrap of ebony which does the trick. All done with scrapwood, but for the next one I'll know to pick a very hard piece of wood as my first choice.
It works beautifully and I can fret a fingerboard in a fraction of the time it used to take with a hammer, plus no dent and no marring...
I must confess I'm inordinately proud of this little contraption! Not having professional tools forces one to devise other ways of doing things and though this entails mistakes and frustrating episodes, it's also a marvellous way to engage the mind creatively.