Paint a base coat; let dry. It typically takes a base coat three hours to thoroughly dry, but follow the instructions on the can. Stencil designs typically stand out on top of a cool-toned base color. Especially when going for brights, use a warm color for the stencil pattern to make it pop.
Mark the center point of the wall where you'll start your pattern. Eye-ball the center or use an extra-long tape measure to find the center. If desired, use a level to mark horizontal lines to follow. Make the horizontal lines the same width apart as the stencil to stay on track and ensure it lines up.
Spray the back of the stencil with a light coat of adhesive. (You might need to reapply after every few stencil paint applications.)
Place the center of the stencil at the center point of your wall, and tape it into place at the level line or level to the ceiling or trim. If leveling to the ceiling, use a measuring tape to measure the distance from the two top corners to the ceiling.
Using a roller, apply paint over stencil onto the wall. Make sure the roller is not longer than the stencil—you may get lines of paint on the wall from the edges of the stencil. Also, be sure to keep the layer of paint thin so you don't have dramatic raised edges between the base paint and the stencil paint.
Remove stencil. Be sure any paint on the stencil is dry, then place stencil for another run, moving down or to the side. This is where carefully measured lines ahead of time help you stay on track. Repeat until you have stenciled the entire wall.