Mix 1/2 cup of mild detergent with water in a bucket. The solution should be sudsy but not foaming.
Wash the garage walls with the detergent using a sponge. Caked-on dirt or grease may require scrubbing. Let the garage dry completely before starting the stencil.
Inspect the wall and determine the stencil design you want. Lay out the design with a measuring tape and pencil to keep the stencil balanced on the wall.
Make the stencil if you don't already have a pre-purchased one. Draw the desired picture on the poster board and cut the center out using the craft knife. The poster board is the frame with the outline in the center, where the paint will be applied to the wall.
Position the stencil on the wall where you want it. Secure it in place with painters tape.
Paint the picture on the wall. Use a paintbrush or sponge to apply the paint. Work from the outside in to keep the paint from bleeding out of the stencil lines.
Re-position the stencil if desired and repeat the securing and painting.