If you don’t need to install a new nut, skip to section 3
Making and installing a new nut
Steps for Making a new Nut.
Acquire a nut blank that is at least the width of the beginning (1st fret) of your fretboard.
Mark the middle of the nut. (use your milimeter ruler or digital micrometer to find the width and divide by two to place your mark spot on). It may be helpful at this point to label the nut’s front face.
Center the nut by aligning it to the center of the fretboard. Again, use accurate measuring implements.
Using a sharp pencil, mark on the nut where the left and right sides of the fretboard end.
Sand the nut at both ends down to the lines marked in step #4. Try to sneak up on making it perfectly flush by sanding, fitting, the re-sanding until it feels smooth on both sides.
With your nut sanded flush to the fretboard on both sides, use a very flat straight edge lain across the lower frets and extended out to the nut and mark a line with a sharp pencil where the straight edge butts up against the front face of the nut. Make a mark on the left and then the right and connect the two with a very precise line. If you have a radiused (curved) neck, you’ll need to mark it carefully at intervals all the way across the nut. If you have feeler agues, stack any combination of them to reproduce the exact height of the 1st fret then use the same stack against the nut to trace a line across its entirety.
Using the marks you made into a line in step #6 on the nut, file and/or sand the nut down to where it meets the line (or a curve for radiused necks). This is called roughing-in the nut and it will give you a great starting point.
For classical guitars, mark a line on the nut for the high E string exactly 13/64″ from the edge of that side of the nut and another at 5/32″ for the low E string. For acoustic guitars, those marks will be respectively at 1/8″ and 5/32.”
Use a file to make a shallow groove dead-on center on the lines you marked for each string. It should be just deep enough for the string to sit in place. In the pic below, notice how different size files are used to make the slots. If you don’t have the ability to acquire a nut slot file set, look for other types of files (such as in manicuring fingernail products) to make slots of different thicknesses that closely match the width of your strings..
Measure the distance between the centers of those two grooves and divide by five. For example, if the distance between the centers of each grove (at the low E and high E) is 45mm, then dividing by 5 will give a string spacing of 9mm. Use this number to mark with a sharp pencil the centers of strings 2-5.
Before filing those same grooves for strings 2-5, make tiny adjustments to the spacings so that wider strings are not compacted. To do this, placing the strings across the nut and with tension added to make them perfectly straight may give a visual aid in making those micro adjustments. Make new marks on top of your previous ones to indicate where the changes are.
With the new proportionally spaced marks across the nut, use a file to make a shallow groove in the same fashion as step #9. If you used the feeler gauges method, you can butt the stack of gauges up against the nut and then file the groove for each string down to the height of the stack, which will act as a safety barrier because your file will not be able to go any lower than that. Place the strings into the grooves and bring up to tension so that they’re in tune. Congrats, you have roughed in the shape of the nut. Now, go on to section 3 to rough in the height of the saddle.