Try this,pick your design,take it to a copy shop & have them print it on "crack & peel" or label {full sheets} stock. Carefully cut out your design usig a new #11 Xacto knife,peel it,stick it & paint it. Don't forget the centers to the O's R's etc.

Check out a product called frisket paper at your local art supply store. It's often used by airbrush artists for masking and creating their own stencils. It has a very low tac, but can provide very clean lines. You could also mask off the area using liquid latex. Simply paint it on to protect the areas you don't want painted, let it dry, spray the 'open' areas, and peel off the dried latex after your spray is dry. Either technique should provide the clean, crisp lines you're looking for.