Design a plan for your Halloween sign on a sheet of paper. Make a list of the shapes and letters you will need to complete your design. Also note the size of the necessary shapes and letters to determine the overall size of the sign.
Draw each shape and letter in the necessary size on tag board or stiff plastic. Cut out the inside sections of the shapes and letters using a craft knife -- these are your stencils.
Place the stencils in position on the sign. Lightly draw the outline of the stencils on the sign to make sure everything fits together correctly. Use movable adhesive to secure each stencil.
Use small amounts of paint at a time, especially if using spray paint, to keep the paint from running under the stencil. The type of paint you need depends on the type of material being used to make the sign. You also may need to periodically wipe the edges of the stencils to avoid paint build up on the edges.
Attach your Halloween sign to a door or wall, depending on your setting.