James Orr wrote:
Lay out the fret slots on paper, then lay them out a second time on the instrument once you know everything is as it should be.
Use double-stick tape on a flexible 6" ruler as your saw guide (one of those $3 rules from the hardware store), then saw. The flexibility will let it arch over the surface of the fretboard.
This is how I cut the slots on my last guitar. It was a multi-scale, so I couldn't use my slotting jig. Sounds intimidating, but it's easy peasy. When a friend teaching a Jr. High cigar box guitarmaking class had 12-14 year olds with absolutely no experience or skill laying out and slotting their fretboards, I knew I could do it, too.
As long as you're cutting along your line, the most important aspects are slot depth and keeping the saw vertical as you cut. A good dovetail saw makes a world of difference.
James, why did you wait until the board was on the neck to slot it? If that's what you are saying.
I have slotted a flat board with just a square. But, already being radiused and already being on the neck seem to add to the challenge. I see what you are saying with the flat metal ruler. Sounds good, but I don't see how I would hold it on there still enough to cut the slots. It's hard enough to slot it by hand on a flat board clamps to a bench!