What we were taught to do, (as I recall) was fill divots, fret slots and the like with rosewood dust, then drop in Crazy Glue. (I think it's referred to in this forum as CA?) Make sure to clean the board or slot with fine sandpaper first. Then set the glue with CA accellerator or baking soda. If you use soda the surface will look dull and rough, but it will finish nicely. Scrape the eccess off with a single ege razor blade. You can actually fill quite a large divot or slot like this. (Be careful to center the fret in the slot.)
As far as hammering frets in, I think the slot should be wide enough that the fret presses (or hammers) in easily. If the slot is too narrow the tang will wedge the board and eventually start backbending the neck. Some people actually grind the barbs off the tang so that the fret just drops in.
At any rate, make sure to bend the fret close to the radius of the board. press or hammer, or drop it in, then hold it in place with a clamping caul and use a drop of CA to seat it. Be sure to use masking tape to protect the side of the neck. The baking soda or accellerator will hold it in place as soon as you use it.
The advantage of doing it this way is that you can adjust the fret until it's exactly in place before seating it. If you pound it in with a hammer or press, you risk having to tear it out if you make a mistake. This is how I was taught to do it way back when at The Apprentice Shop in Spring Hill Tn where I took a guitar repair course. (They're the Gibson "factory repair" shop.)
I haven't worked as a repairman, just on my own guitars, but this method has worked well for me.
Good luck!