Fretting seems to be one of those things about building that a lot of folks dread....I find it to be one of the best parts of building along with carving the neck; both are very rewarding upon completion. In the moment, I will sometimes wonder why I don't just take a Sharpie, mark the frets with it, raise the action, and play it with a slide. But after a good fret job, I feel great. Here's the way I do it and some things I've found to make it easier. If you have any other tips, please let me know.
I used to use a variety of scale lengths for my CBG's. I was all over the place. And that way lead to madness and frustration. I've settled in on a 23.5 (Byrdland) scale, 20 1/8, and a 17" scale. The latter two are Ukulele scales. I have yet to make a Uke but will be ready with those two options when that day comes. Because of the antique boxes I use, I prefer these smaller scales. The guitar seems to have a balanced look; rather than having a long, long stick jutting out of a tiny box. The madness part of too many scales comes from selecting (and buying) strings for the variety of lengths. String gauge needs to be matched up with scale length. I went thru a load of strings trying to get them to sound right up and down the scale. What sounds good at the top of the neck does not stay tuned as you proceed down the neck. Here's the string tension calculator I use: http://www.mcdonaldstrings.com/stringxxiii.html
A great scale calculator can be found (like the above; for free) on the Stew Mac site: http://www.stewmac.com/freeinfo/Fretting/i-fretcalc.html
I put frets on very early in the build. Once the neck stock is sized, I begin to layout the guitar. Measuring from the top, I measure the length needed for the headstock, locate the nut, mark the distance from nut to bridge, and then figure where the box top and bottom will be. I tend to put the bridge about 1/3 the box length up from the bottom. I read that that was a good position and gives the best resonance. I may have dreamed it to, who knows.
I'll then cut the angle for the headstock, make the cuts for the box and cut the layback angle for the box. Back on the bench, I'll begin to lay out the fret position. I use and recommend layout using millimeters. It is far more accurate and you do a lot less math: what the heck is 3.047 inches and where did you get that ruler?
I tape the mm ruler in place and begin, making only one tiny mark for the fret position using a very sharp pencil. Sharp pencils are really important. Mechanical ones work well I suppose but I love using my antique Boston pencil sharpener. Using either my homemade mitre box or the Stew Mac version (whichever is closer), I begin to cut the frets. On the stew Mac one (an older model, I have to use a couple of shims (cut and thinned to the exact same thickness) to move the neck away from the wall. The little brass blade guides will get in the way. The mitre box is designed for thin fret boards to fit under these guides, not for thick stock CBG necks. Good point though: I use fret boards every once in a while, mostly I add frets directly to the neck, and a lot of what I'm writing will apply to separate fret boards but only after gluing it to the neck stock.
Checking the marks, I cut the frets. I don't use depth gauges or stops on the Japanese backsaw (another Stew Mac investment but an important one for tight frets). I use the teeth as my guide. Saw to the depth of the teeth. If the teeth are not visible in the cut, that's deep enough.
I like to lightly sand the fret board once all the frets have been cut. With sandpaper glued to extremely flat scrap thick wood, I will sand lightly from 120 to 400 grit and top off with steel wool. This will give the fretboard an incredibly smooth feel for fingers later. It's also the last chance to prep the fretboard before adding frets. At this point you could add the frets but I taper the necks so I wait till after gluing on the headstock (angle and wings) and gluing on the brace under the box cut out for strength and stiffness. The neck taper is cut after all that.
Its a lot of back and forth in the process between the garage and the basement. My shop is in the basement and the saws are out in the garage. My basement floods every once in a while and the thought of my low mounted bandsaw motor under water just brings tears to my eyes. In case you don't know it, the bandsaw is the single best machine you will ever buy.
Now I add the frets. Clean out any dust in the groove from sanding. An old paint brush does a fine job of that. I used to re insert the fret saw and clean it out. What a waste of time that was in the past. Be sure there is support under the neck and it is held firm with clamps to you bench top. The frets tap in way tighter and eaier when the neck is firmly in contact with a solid surface, this is really important. Lay the fret wire on the leading edge of the fret and begin to tap it in as you tilt toward the far edge. Trying to hammer it in flat doesn't work well; fret grooves gets damaged or you'll dent the fret position. Let the fret wire hang over the far edge just a bit. Tap it in and trim off just the front edge. Cut off the other side later over a trash can so little tiny fret wire cutoffs don't end up on the bench top where the inevitably get pounded into the neck by accident.
Once all the frets are in and trimmed, put the neck in the vice (add wood to the jaws if you haven't already) with one side facing up.The neck should be above the jaws of the vice slightly. Use a medium to fine flat file and slowly and carefully file the fret wire even to the side of the neck. This is one of my new discoveries, may be old hat for a load of you. This works great and is very fast. Once it feels and sounds like the frets are even to the side, continue filing but begin to angle the file toward the fretboard. On my vice I can get about a 25 degree angle before hitting my knuckles on some part of it. Angle as best you can. Flip and do the other side.
I have a piece of wood that I cut a 35 degree angled groove in and added a flat file using hot glue to hold it in place. This is another tool available from Stew Mac but at some point you just have to say no and build your own tools. I love the Stew Mac but they are really expensive!! Notice they never ever have a sale!? Once both sides are filed flat and angled a bit, reposition the neck in the vice so the fret board is facing up. Now I use my angled file and finish the beveling. Finish up with a light sanding of 220 sandpaper (on the bevel not the top of the wire) and then some #1 steel wool.
That's it. Feel it. You can smooth it out further using 400 grit and finer steel wool but you know you are going to tweak it later anyway. Put the neck away and be happy with a nice job.