Step 1: Get an image
Step 2: Prepare materials
If you are using a found image (or making a copy of someone else's stencil) you will want a black and white copy of that image and a few glue sticks. Flip the image blank side up and smear some glue on it, you want to cover as much of the surface as you can with a thin layer of glue. There are also spray adhesives for things like this, but they are not available at the local dollar store.