AlanRatcliffe
You need a modified flat metal file and a small flat keyfile (also called a watchmaker's file - usually sold as a cheap set of miniature files in a plastic pouch). The modified file should usually be a 2nd cut, but you can use a bastard, it'll just cut faster and need a bit more smoothing afterward. This file also works great for fret levelling.
To "modify" it, you must cut an inch or so off both ends (as they are usually not properly flat at the ends). To do this: using a grinder, score across the face where you want it cut, clamp the end in a vice and yank it until it breaks along the scoring. I epoxy a piece of wood down one face to give me an improvised "handle".
You don't have to remove the neck, but if you can it makes things easier. If it's a set-neck, mask of the body around the fingerboard thoroughly with two layers of tape and also mask off the neck pickup, to prevent filings from sticking to the poles.
Slack off the trussrod completely and check that the surface is straight with a steel ruler on the frets. If it is slightly concave, tighten the truss until it is flat. This ensures that you cut all the frets evenly.
Now you can use this file down the sides of the fingerboard, running it along the length of the neck, cutting from first towards the last fret (if you cut the other way around, you will ding your headstock). Cut at an angle to angle the edges of the frets. Make sure that the angle doesn't intrude too far on to the fret surface itself as you'll shorten the effective fret surface and make it easier for the strings to pop off the end of the frets and off the neck.
Cut down the frets until you start cutting in to the fingerboard itself - this is a good thing as it angles the edges of the board (or binding) too, giving them a more played in worn feel that's comfortable to the fretting hand. If you have a maple board, bear in mind you're going to need to touch up the finish to seal it after doing everything.
Do not run the file down the sides of the neck as this damages the finish. Usually the fret ends will be under the finish on the sides anyway, which makes them smooth to the hand.
Now take your key file and smooth off all the corners and edges. Try not to mar the fingerboard - mask off between each and every fret if you have to. I have thin metal fretboard protectors with slots cut for the strings - nice, but not essential. If the fret ends protrude through the finish on the sides, use the keyfile to level them with the surface, but once again, be aware this will damage the finish.
Now use some 600 grit wet or dry paper (just around your fingers, no need for a block) and swipe it along the bevelled edge for a minute or so, similar to the way you did with the file. This smooths of the edges even more and softens the angles a bit. Do the same with 800 and then 1000 grit. Run your hands along the edges to make sure all is smooth and pay attention to any remaining rough spots.
Dust off any and all debris - compressed air is best as it reduces the chances of scratching the finish with debris caught in a polishing cloth. If you masked off the neck pickup on a set-neck, stick another layer of tape over the masking to trap the debris before you remove the tape as one layer. Clean up with lemon oil (rosewood, pao ferro or ebony) or polish (maple).
Hope that's all clear...
