Finally! Something I DO know about!
I use "for sale" signs... The thinnest ones you can find (which are still way heavier than card stock).
Get a really sharp xacto knife.
Get a can of spray adhesive. I use 3M.
Print your image on copy paper. Glue it to the for sale sign with the spray adhesive. Cut it out.
Here's the real trick! Spray a light coating of spray adhesive on the side of the stencil that will be in contact with your bullet boxes, BUT LET IT DRY WITHOUT STICKING IT TO YOUR BULLET BOX!!! This will allow the adhesive to get tacky, but will remain removable.
This will give you complete contact for your stencil.
Now, when actually spraying your stencil, use several thin layers. Letting them dry between applications. This will give you the cleanest image.
TA DAAA!!!
also, check out stencilrevolution.com.
That's where I learned the most.