Tom, this may help your neck building?

https://youtu.be/VN5yNCWMr_E

another PS:  don't try building a mando until you have made a couple of successful fretted CBGs.  Mandos are tough to build and have play and sound good.  Really the shorter the scale, the harder and more exacting to have it come out good.   Lots of folks buy a pre fretted fret board.  Good way to go and they usually allow you to use a bolt for the nut - but that bolt or screw must be precisely placed.

Or do as below a fretted 3 with DGB tuning make easy playing in the key of G (G is open) and 2 fingered C and D chord and 1 finger Em--- plus you can still slide it.  Nice to have both finger chord CBGs and slider CBGs.

I am a mediocre player and only use the first few frets so sometimes that is all I add frets to.  

I see that you found knotlenny and have the how to play thing from the blues box.  Hey, if you already play guitar and mando, build yourself a fretted 3 or four string.  Best way is to copy frets directly off a storebought guitar onto you fret board.  Use a 'fret' for the nut - that's called a zero fret.  Then use some kind of string guide - look on CB Gitty for string guides and or look at a lot of pics.   Use the 4 small strings from a guitar and tune DGBE - plays like a six minus the big two.  Also plays like a uke - same string relationships and fingering, but different chords come out!