Some tips from a painter - BTDT
Stencils are cut from low-tack vinyl. The adhesive is less tenacious than sign vinyl so it can be lifted off. If you bring a positive image (black decals) to a sign maker he can scan them. Someone (either you or the sign maker) has to "pick" the inside parts of the images, which is not fun (magnifying glass and tweezers), then cover the stencil with a backing sheet. Buy more than you need, you WILL screw up and get wrinkles the first time.
Paint the parts including one good coat of clear and let it cure overnight. Wet sand 800 or 600 grit before applying the stencils. You need some tooth for the stencil paint to stick to, else the paint will lift on the edges when you remove the stencil. Mask everything that won't get painted - there's a sign on my shop wall that says, "Even airbrushes make overspray" as a reminder of my past mistakes.
Remove the stencil while the paint is still wet to minimize the chance of paint lifting and cracking at the edges. You'll need very steady hands and good tweezers to remove the centers of the Norton and Commando "o's", "a" and "d" without damaging the paint underneath or leaving ragged edges. That's another case for the buffer coat of clear - if you screw up you can clean the stencil off and start over without risk to the underlying paint.
I've had good luck using positive images as stencils. Example, black and gold. Spray gold as a base, plus a light coat of clear. Then apply positive decals and paint black over them. Remove the decal and gold is underneath. This also works well for pin striping.