I have been asked many times: How do you get the frets in the right place? Well, guesswork isn't good enough. There's a simple algorithm to figure it out though: take your string length, divide it by the twelfth root of 2, subtract that from the original length. Then repeat twenty more times or so...
The biggest problem (aside from tedium-induced narcolepsy cause by all that calculating) is actually in transferring those numbers to the physical fretboard without errors: they're cumulative, and the measurements can go off course in a hurry. My favourite solution was to print out a simple template generated by Jon Tirone's free program wfret. It prints out lines with the correct spacing, no cumulative error, and you just tape it to the side of the fretboard to know exactly where to cut the fret slots. It gets cumbersome when I'm making a guitar with different scale lengths on different strings (a "multiscale" instrument), but worse, it only works on windows.
Then just yesterday I found this: Aaron Spike's FretFind2D
Web-based (java), so it's free and works on any system (within reason), it creates a full representation of the fretboard, all the way to the bridge if needed. I can make a template of a regular or multiscale guitar, with as many strings as I want, I can specify the string spacing at nut and bridge, fretboard overhang (independantly at nut and bridge), I can even specify Just Tuning or Equal Temperament. And when I'm done I can print it out as a full-size PDF split appropriately across multiple pages; or as a DXF if I had a plotter. Oh yeah, and it's so easy to use I had it figured out pretty much instantly. Crazy Awesome.