Choose your box
First, choose your box: cast aluminum (Hammond or Tawainese 125C), or electrical (steel) junction boxes seem to be the most popular. If you use a junction box, either braze (with a torch) or epoxy (with JB weld) the knockout plugs ...or just don’t mount anything on a knockout plug!
Draw a sample layout to make sure all the hardware will fit on your box, or use a pre-made template.
Drill the holes
Use the center tap and a hammer to mark each hole. (St. Louis pod: put the drilling template over the box, and use center tap right through the paper).
Drill the anti-rotation holes
These are tiny holes that a tab on the pot fits into, keeping the pot from rotating and coming loose over time.
Take a pot and put the shaft in the box from the INSIDE. (make sure the anti-rotation tab is sticking up).
Choose how you want to orient it, and then rotate it slightly to scrape the anti-rotation tab against the inside of the box. The scraping should leave a faint mark.
Repeat for each pot
Use a small bit (3/64”? depends on your pot type) to drill a hole along each scratch mark.
Note: Anti-rotation holes are completely a matter of preference. For normal use, lock washers are usually sufficient. You can pop off the anti-rotation tabs with pliers.
Smooth out the holes Lightly go over the holes with a 1/2” bit to round the edges down (countersink). If the hole isn´t smooth around the edges, the washer and nut wont fit straight, and it will crossthread and ruin the pot/jack. So maybe have some pots and jacks with you when drilling to test your holes out...
Tip: Use cutting oil if youre drilling steel boxes. Also, if you plan to do a lot of this, a good-enough drill press can be bought for under $100: much, much easier on the hands.
Next: Painting