Starting point - The generic plastic supplied nut.
First - remove the old nut. Use a block of wood and gently tap the nut in all directions until its loosens. It will then lift away.
Disregard the removal of the machine heads (shown in this image), this guitar is having other modifications as well.
We need to know a few things first before we can make a nut. The first - What is the fretboard radius? The fretboard is a Number 16. The '16' stands for a 16inch radius.
The next thing we need to know is the width of the fretboard. At the end of the pictured fretboard, this is known as the "nut slot width", it is 41.99mm or simply 42mm. I am using a set of verniers to measure.
We need to know the size of the strings we are going to fit. In this case, I am going to use a set of Ernie Ball Slinky's, as supplied with Kit 1. The Slinky's are stock No 2221. The string gauges are 10",13",17",26",36" & 46".