Guitar fretboards are generally curved, so when fretting a fingerboard it helps if the frets themselves are also pre-curved (to a slightly smaller radius than the fretboard itself) before you try to put them into the fingerboard.

You can buy fret benders from Stew Mac, but this tutorial from ZDguitars.com shows how you can make your own. You will need some skateboard bearings, cap-screws. washers, nuts and a piece of wood. The tools required are: a file or sand paper, a saw, a drill press or hand drill and spanners for the nuts.

Link: Make your own fret bender