Step 1: Materials
Self-adhesive vinyl sheet (mine was about $1 at Hobby Lobby)
Cardstock or file folder (optional)
Step 2: Design
I started by copying a picture of my bass of the internet and drawing over it in photoshop. Then I took that design and drew it on paper. Getting the pickups and volume/tone knobs to line up took a lot of trial and error. It helps to place the paper on top of everything and tracing where things are located. I went through at least a dozen different paper templates.
I recommend using heavier paper after you get the design down, as it's easier to keep it steady as you get all of the details to line up.
Step 3: Cutting the vinyl
Use a pencil to trace the paper template onto the vinyl. Cut the outsides with scissors and use an exacto knife for the insides. Cut out slots for the pickups but not the volume/tone knobs.
After making sure the slots line up with the pickups, check where the knob shafts are. Mine shifted a bit between tracing and cutting, so I had to redraw the holes before cutting.