I looked on all the Instructs tagged with stencil, and not one gives a step by step for making a stencil in my favorite open source photo program The GIMP.
Step 1: Materials and GIMP
-surface to cut on (self healing mat)
-spray mount (optional)
GIMP is an open source raster based graphics editor. It was designed to replace Photoshop.
You can get it here. gimp.org
Install it, open it up, and familarize yourself with the interface for a bit. Done? Good. Isn't open source great? Didn't cost a dime!