... set up the rip fence so that the first slot was down the center of the block. Then we edged the rip fence in a little less than a blade-kerf width, cut the block in one direction, then flipped it and cut it in the other direction, so that the cut stayed centered. We repeated that process until close to the slot width we wanted, then kept going, but then edging the fence in in in tiny increments until the slot was just the right width ...

I thought about building one from plans but ordered Jon Simpson's which comes with both Dovetail and MT joint templates http://www.jsimpsonguitars.com/neckjig_sales.html
It has worked well for me for the last few years, by the time I bought all the materials and the time to build, it was a great investment.

Fred

The one I have is several years old and he has improved it some. My only change would be to make the templates adjustable so the length of the dovetail could be changed to match different body depths, but it works fine as it is. Jon is real great to deal with and is helpful if you have questions.

Fred

The "table" is basically slotted, so the template is captured on center but can adjust for tenon depth. (The trussrod slot keeps the neck blank aligned & on center with the template.)

All of these jigs are variants of Paul Woolson's jig, for which plans can be had from OLF. Provided you have some skill in jig building, it's a pretty simple job.

Robbie's plans lack some crucial measurements, and his design doesn't address a number of common features. Just to call a few out, he has you cut out a 2.5 wide opening for the body?tenon or dovetail. The problem here is that if you have a cutaway you're only going to have .125" of body resting on this opening on the cutaway side of your body. A more prudent opening would be to make it wide enough for the dovetail of a reasonable depth, and taper the opening accordingly. This way you have the bottom of the opening supporting the body as it is in the clamp.

He neglects to recess the tee-nuts for the templates for the body, consequently, the tee nuts and bolts end up against the body. The lack of the center finder on the body clamp is another problem. Just because the centerline of your dovetail or mortise is centered within the template doesn't mean that the body itself is aligned with that centerline. Consequently, you can end up with the dovetail or mortise angles off the centerline of the body. Another problem is that it assumes that guitars are built within 90° angle between the top and the sign with the neck joints. A better solution would be to cut matching 1.5 degree angled cuts on the body clamp top edge as well as the top of the fixture to have so that when the body is clamped it sits flush against the client and against the template.

Guitar backs are both arched and tapered yet his design uses a 2 x 4 uncompensated for this reality. A better solution would be to arch and taper the body clamp according to the relative taper of the guitars that people generally build.

In his video, his overall lack of attention to exacting cuts and alignment is part of the problem.