I've always noticed that a truly perfect neck joint is one of those things that very few amateurs get right and even a fair amount of respected pro luthiers struggle with as well. For whatever reason, neck fit has always been very easy for me and I credit that with the process I stumbled upon rather than technique -- because there were loads of things I struggled with early on as a builder.
Here's my thought on neck fitting:
1) The upper bout "ramp" must be perfect. The upper bout area beneath the fretboard must be perfectly flat and it must create the perfect neck angle -- there are several ways to achieve this but I use a sanding board with a properly sized shim at the bridge location and sand the upper bout until it is level.
2) Here's the big one -- the sides must be perfectly flat/level across the width of the sides. This must be maintained during the finish process. A hard sanding block is absolutely necessary to achieve this and it needs to be within 0.001".
3) Undercut the cheeks of the neck heel so that is easier to sand. I use a combination of 3m stickit paper and regular sandpaper backed with tape (to avoid scratching the finish). Go slow and methodically and achieving an absolutely flawless neck joint is very doable with relatively little skill. Trust me -- if I can do it, then anybody can.
