Create a stencil shape by cutting a design out of thick cardboard with scissors or a utility knife, or buy a stencil from a craft store.
Mix the spackle according to the manufacturer's directions. Typically, ready-mixed joint compound only requires gentle stirring, but setting-type joint compound requires you to add clean water to the powder to achieve the proper consistency.
Hold the stencil flat against the surface you are stenciling.
Use a drywall knife to apply a smooth, even layer of joint compound across the stencil. Choose a knife that is wider than the stenciled design to minimize lines from wiping. For example, if the stenciled shape is 5 inches wide, use a 6-inch drywall knife.
Peel the stencil away. Move slowly and carefully to avoid smudging the design.
Allow the compound to dry according to the manufacturer's directions.
Sand the design gently with 150-grit sandpaper if there are any surface flaws or lines.