Step 1: Start with a Stick!
I use "instrument-grade" wood such as mahogany and black walnut. Once I built one out of a African wood called bebinca. All our wood is donated to the Andy Mackie Music Foundation, for the purpose of teaching children to make and play their own musical instrument.
If acquiring these kinds of wood is just not possible, heck, then try using what is available to you, such as ash or another type of non-instrument-grade wood. The reason I suggest ash, is that it may bend more easily in a future step. Experimentation is most encouraged!
Here the measurements can vary. If you want to make a wider one to accommodate more than three strings, that will work too! Our kids have made many with varying lengths, including 12 inches longer than 30.5 inches, as well as shorter "ukulele-sized" instruments with less of a fretboard range.
Noting the direction of grain is important, such that when it is time to cut the fret wire grooves, the saw slices perpendicular to the grain.
Step 2: Measure the slope
Here we're measuring the slope, where the tuner-holes will eventually be drilled.