One approach to customizing a guitar is to mix a variety of colors and designs. Eddie Van Halen popularized this style in the 1980s by spraying stripes on his guitar. Sketch out a color scheme or patten on paper and use this as a guide. To prepare the guitar, sand off the old finish. Apply a primer coat and lightly sand the guitar after it dries to create a smooth surface. Using acrylic spray paint, put down a base coat. Let it dry, then apply a second coat. Apply the additional colors and designs. Stencils work well for painting intricate designs or patterns. After the paint is dry apply a few coats of clear to protect the finish.
Guitarists often customize their guitars to match their particular genre of music. Skulls and flames are the most popular decorative effects used by metal players. Typically the background color is black or a dark color. This can be applied with acrylic spray paint. The outline for the flames and skulls can either be drawn on the guitar or applied with stencils. The artwork is often intricate and requires detailed brush work. One variation is to use purple and blue for the flames rather than orange and yellow. It is helpful to sketch your ideas out on paper before painting them on the guitar.
A third option is to recreate a famous work of art on your guitar. For example, mimic Andy Warhol's infamous neon portraits of Marylin Monroe. Either use Marilyn's face or appropriate Warhol's style for another subject. Impress your girlfriend by painting her image on your guitar. This method of customizing your guitar requires a little more artistic skill than the first two approaches. If you are not artistically inclined, find an artist friend to help you.