Unwind the tuning pegs on the guitar with your hands, and remove the strings.
Remove the screws securing the neck to the body and those in the scratch plate with a Philips screwdriver. Sometimes the scratch plate is soldered on; in this case, carefully tape over the scratch plate to protect it.
Protect the surfaces of the guitar you don't want paint on, such as the neck, with painter's tape.
Sand down the surface of the guitar with medium-grit sandpaper. Move in long, even strokes. Then upgrade to a fine-grit sandpaper and repeat the process.
Wipe down the surface with a damp cloth to remove the excess sand grit. Then allow the guitar to dry thoroughly.
Apply primer to the surface of the guitar with a paint brush. Use wood primer if the guitar is made from wood. Allow it to dry. Then turn it over and paint the other side if desired.
Apply one to two additional coats of primer. Allow each coat to dry before applying the next coat.
Place a protective mask over your nose and mouth, and goggles over your eyes.
Apply a flat or glossy latex spray paint to the guitar. Spray paint will give you an even surface, free of brush strokes.