At first you need some guitar. :) And just some creative idea.
Then you have to buy some adhesive film, it is just sticky in one side (after you take of second layer).It's the best choice, because if you bother it someday you can take it off. Of course, you can just paint your guitar with gouache or something like that, but it won't look as shiny (or pretty) as in this project. :)
At this step you have to take a pencil and draw a sketch of your wanted creation. Then you have to draw (or put your sketch on it) pattern on adhesive film.
Then you you have to gently cut your pattern on adhesive film with scissors. Be extremely careful :)
A this step you have to take of a non-sticky layer of adhesive film, and put it on your guitar.
And your guitar now looks totally more creative. :)
P.S.Don't put too many adhesive film all over your guitar, because then your guitar won't sound very good.