I have a 1986 VW Vanagon Syncro... the coolest part of which is the locking differential. I decided that the stock knob with which the differential is engaged was lacking in personality.
Step 1: 1. Measure your threads.
Retrieve the thread gauge from your Tap and Die kit. The thread gauge looks like a little keychain of tiny saws with different size teeth.
We want to figure out the thread measurement that we're going to recreate on our new part, and the way to do that is to try fitting different gauges until one matches perfectly. You will feel this instantly, and there won't be any question if it's the correct match- it's a powerful feeling of rightness, like a double rainbow over an ice cream cone of satisfaction. With sprinkles.
There are metric and english thread sizes, and if you can't find teeth that match your threads on one gauge, you're probably on the other measurement system. Metric threads are measured from peak to peak, if you look at them sideways. English threads are measured by number of peaks or threads per inch.
The gauge will have the thread information printed on it, allowing us to select the proper tap for the job.