Scrub the patio floor to remove any dirt and loose pieces of concrete. The paint or stain will adhere best to a clean, smooth surface.
Tape off the area where the stencil will go. Cover the rest of the patio and any surrounding areas with sheets of plastic secured with masking tape. If you plan to spray the stain, be aware that tiny droplets of stain often become airborne and settle on the floor a few feet away, creating a thin haze on other parts of the floor. To avoid this, cover the surrounding area for several feet in each direction.
Apply one or two coats of a concrete primer such as Skimstone to the area you plan to stencil. The primer will create the desirable texture and allow the stain to adhere properly to the floor. If your stencil pattern needs a base coat, you may be able to have the primer tinted to eliminate a step in your painting process. Allow the primer plenty of time to dry before proceeding to the next step.
Heat the area with a heat gun for about 5 minutes if the weather is cold. Stencils don't always adhere well to cold concrete, so heating it will improve your end result.
Stick the stencil in place very carefully, being sure to smooth out bubbles and air pockets as you go. Nearly all concrete stencils come with an adhesive backing for easy attachment. If you plan to use a stencil without adhesive, tape it into place thoroughly.
Tape off any sections of the stencil that you don't want to fill in with the first paint or stain color. Be sure each area is taped thoroughly.
Roll, brush or spray your choice of coloring into the stencil. You may need to use your fingers or a small paintbrush for very small areas. If you're applying paint with a sprayer, you may wish to overlap several colors during the same application.
Allow the first coat of paint or stain to dry thoroughly. Remove tape from unpainted sections of the stencil and tape off any areas you don't want to paint with the second color.
Apply a second paint or stain color, if desired. Repeat the taping and painting process until your pattern has been completed.
Wait for the paint to dry fully, according to the manufacturer's instructions. Paint that is not completely dry may peel when you remove the stencil.
Peel the stencil off the concrete. Because many stencils have very intricate detail, the stencil may break or tear in a number of places while you are removing it.
Finish your stenciled patio with one or more coats of weatherproof concrete sealant.