Keep in mind the fretboard is already glued to the neck. So Should I carve the back of the neck first, or hammer in the frets first to keep the straight edge of the back of the neck? Would hammering in the frets with the neck already carved be too much of a problem?
Larrivée D-03R
US Fender Telecaster, Vox AC15
It really doesn't matter.

Edit: If you have the size set, then it doesnt matter. But if you are going to cut down the neck some to make it thinner, than you should do that before you do anything.
Growing old is inevitable, growing up is optional

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Custom Build
You can do it either way. If you carve first you have to use a caul to support the neck.