http://www.stewmac.com/Luthier_Tools/Types_of_Tools/Files/3-Corner_Fret_Dressing_Files.html See the video here as well. You can get by for a lot less money than what they ask for these files sold as for fret work.
http://www.stewmac.com/How-To/Onlin..._fret_ends_that_stick_out_in_dry_weather.html
Or an inexpensive file, (one, or two files), or a set from the hardware store like any of these:
miniature file set - Google Search. and some masking tape will work too.
Files with handles, like these:
12 Piece Mini Metal/Diamond File Set - Garrett Wade help prevent cramping of the hand.
Diamond files work well for fret work.
Start with a coarse cut, and finish with a fine cut file.
A file with smooth, also called "safe" edges will help you avoid damaging the wood, if you can find one.
There are different "cut" and "pattern" types of files.
Hand File Identification - The Tool Corner Swiss pattern files are probably better than American pattern files for fret work. Diamond files eliminate the task of choosing a cut, and pattern type.
Go slow, take your time. Do it in good lighting, and if you use a head-worn magnifying lens of some sort, you will be able to see how close you are getting to the wood without going too far. If you can remove the neck, or at least brace the instrument with the work oriented upward so you can see what you are doing, and find a spot to brace your hand or forearm so as to steady it and maintain better control, you should be able to get very good results.
Finally, polish with micro-mesh pads to get professional shiney looking, and smooth feeling results.