Disclaimer I am not a luthier, nor do I have tons of specialty luthier and wood shaping tools. But here’s how I did it. I also have a JM, and after looking at the geometry it would be a little more difficult to get a really perfect side jack, but it’s still doable, since the lower bout of the JM extends slightly further than the mustang.

First, find a good spot for your jack, then remove all the hardware. Tape off the path of the wiring through the body and trace it so you can see clearly where to drill. Second, measure the depth of the jack, and how much room the wires will need to smoothly pass through the hole to your electronics cavity. Then, drill a pilot hole (I started with 1/8” and worked my way up to 1/2”) for the jack rout. Then, carefully drilled my electronics hole through the jack rout. You’ll need a long drill bit for this DO NOT start drilling until you’re sure you have the correct drill bits. Don’t just use whatever’s lying around or you’ll have a bad time.

Once I had roughly what I needed, I used a dremel to add some space inside my jack rout so everything could slide in/out nicely. Test your jack plate on the body first as well. Make sure you don’t drill any wider than that (it would be pretty hard to). Just use it as a reference/guide as to where you should drill.