Cut letters or shapes out of cardboard. Hand cut, die cut, use a personal cutter, whatever works best for you. One way to do letters is to print out the letters you want in a font you like on your printer, then use them as templates.
Then paint on a thin layer of white paint. Next drizzle with 3 primary colors. Add white with some silicone on top. Swipe with a palette knife. Be sure it goes all the way to the edges.