These are pointers to help maximize your success in installing a shelf nut. Hmm, my readers have a wide variety of knowledge and skill - I hope I can strike a balance here, and emphasize the things unique to nut compensation. Safety note: always use a mask when cutting or sanding bone material!
Estimate your compensation needs, before ordering
If your original nut was accurately placed at the "calculated" position, and your action at the nut is not too high, you should find that the 1st, 3rd (in wound), and 4th strings are only off by about 0-3 cents and that the 6th and 3rd (if unwound) are off no more than about 9 cents, then this shelf nut should be a good choice.
If your original nut has been set forward, you will see overcompensation on the 1st, 3rd (if wound) and 4th (the pitch will be flat at the 2nd fret, compared to open.) In the case that only one or two strings need positive compensation, you may prefer to just glue bone extensions to the original nut, and file back the string release point for the overcompensated strings, as I have previously done.
Verify that your fretboard radius is correct, before ordering
Ideally you will have a fretboard radius guage to determine the radius of your fretboard. These are available from a luthier supply, such as Stewart-MacDonald, but also from Amazon or ebay. There are several types to choose from.
If you expect this to be a one-time use, you could even make your own radius guage: take a thin stick, pound a small nail in one end, measure from the nail to the suspected radius and drill a 1/8" or 5/64" hole. Insert a pencil firmly into the hole and draw a 2" wide arc on a piece of cardboard; cut it out carefully, and that's your guage; if it doesn't fit, try another radius.
You will need to loosen or remove the strings, and apply the guage to the fretboard near the nut. (Some guitars have a compound radius, so the radius may be different up the board. Also, your fretboard may have been resurfaced; resurfacing parallel to each string path causes compounding - widening the arc going up the fretboard.
I found, on my guitar, the fretboard was just a little more curved at the nut (smaller radius) than advertised. If that is the case, for you, then you will need to do a bit of sanding on your fretboard to make it match the guage (and the nut). finish with fine steel wool and apply a little bit of oil (mineral oil or olive oil works great). My fretboard was even more rounded at the edges of the fretboard, which resulted in a small gap between the fretboard and the shelf at the ends of the nut. It was not enough to be a problem, the shelf being solid with the the rest of the nut, it doesn't need to actually touch the fretboard.
Care and patience for adjusting the bottom of the nut.
After sanding off the nut width to fit in the slot, the next step is to trim the bottom of the nut. It has to be just right in order to rest flatly on the bottom of the slot while the shelf rests on the fretboard. It's OK to machine sand or cut off excess height, but the final sanding should be done by hand. I use 100 grit sandpaper on a flat surface.
Be very patient, and try to prevent over-shooting, which would necessitate shimming. Because the string slots have not yet been started and the ends have not been trimmed, you can move the nut from side to side to best fit the fretboard curve.
Try to prevent any rocking motion when sanding, which would produce a curve in the bottom. To remove any curve, I sand over the edge of the sandpaper, starting near one end and stopping before the other end. By changing direction and sighting the bottom edge, you can make the bottom perfectly flat.
When the nut bottom and shelf are a good fit, the nut can no longer be moved sideways, so mark the ends well, so the your nut slot starts can be properly marked.
If you haven't chosen a string spacing method, there are three choices: 1) equal spacing between string center lines, 2)equal spacing between strings, and 3) proportional spacing - spacing increasing evenly by adding 04" to each space from treble to bass.
Probably the most popular choice) would be proportional spacing. You can accomplish this with the aid of a String Spacing Ruler, available from a luthier supply, amazon, or ebay. The string spacing ruler from Stewart MacDonald adds .004" to the space as you go the next lower string.
I personal prefer equal spacing; you can calculate* the length of the space by dividing the distance between the low and high E strings by 5 (there are 6 strings, 5 spaces between). Some people have two-pointed dividers to lay out the string positions. If using a ruler, for each string position, measure from one of the E strings the sum of the space lengths. (If you measure each one from it's previous, you will probably accumulate too much error.)
My next choice (probably the most popular choice) would be proportional spacing. You can accomplish this with the aid of a String Spacing Ruler, available from a luthier supply, amazon, or ebay. You calculate proportional spacing**.
Adjusting nut action height while adjusting compensation
Initial adjustments can be made to the compensation offsets based on your experience, on strobe tuner measurements, and, perhaps, use of the paper clip tool. We know that the 1st and 4th strings will require little or no compensation and the 6th and unwound 3rd will require a lot. The others - somewhere between.
Proceed by installing the nut and strings, and recording strobe tuner measurements on all the strings - cents off at fret 2 compared to open. Loosen the strings the minimum amount needed to remove the nut (on an acoustic steel-string, that would be enough to pull the bridge pins). Then file your trial-and-error adjustments . Repeat these steps until you have succeeded. (I usually do this with used, but not badly worn strings; strings tend to break at the capstan from repeated tightening and loosing.)
You cannot finalize the correct compensation offsets until the nut action heights are also finalized, since these adjustments affect each other. The nut slot slopes downward toward the peghead; therefore reducing the compensation offset also reduces the string height. To minimize this affect, make your nut slot angle very slight until your offsets are very close to final.
Finish Nut Slots
File the nut slots at a minimum downward angle of 4 degrees. If your guitar has a shallow peghead angle (like Fender) then angle the slots no more downward than toward the peg tops (if the angle is greater, then the strings will bear only on the forward edge of the nut, resulting in rapid slot wear and, perhaps, string breakage.) For guitars with greater pegboard angle, I recommend a 6 - 8 degree downward angle. Keep the slot angle steady while filing, and make sure that the slots do not taper downward at the string release points. Then at the rear part of the nut, curve downward toward the capstan. Check that the slots are wide enough and are well rounded so that the strings don't bind.
Don't expect perfection
While strobe tuners can internally detect pitch to .1 cent accuracy, don't expect your guitar tuning to be anywhere near that accurate! It's difficult to tune a guitar even within a half cent, and a half cent difference on the tuner is barely noticeable on a strobe tuner. Try for .5 cent accuracy on each adjustment, but be happy with 1 or 2 cents off in some places on the fretboard. It will still be a big improvement!
*Calculate Equal String Spacing
Even spacing means that the spaces are all the same, center to center. First determine your desired span from the first string to the sixth, center to center.
span = space x 5
calculate: space = span / 5
**Calculate Proportional String Spacing
With proportional spacing, spacing increasing evenly by adding 04" to each space from treble to bass. First determine your desired span from the first string to the sixth, center to center.
(Skip two lines if you don't like math.)
span = (space1 x 5) + .40"
space1 x 5 = span -.40"
space1 = (span -.40") / 5
calculate: space1 = (span - .40") / 5
calculate: space2 = space1 + .04"
calculate: space3 = space1 + .08"
calculate: space4 = space1 + .12"
calculate: space5 = space1 + .16"