The easiest way to do it, but not the most inexpensive, is to get the mask done for you. Basically you want to first paint the bottom of the truck the darker color, then mask off what would stay dark and then spray the lighter part.
So you go to a place where they make vinyl stickers and lettering. Some malls have little carts that do things like that. Get the kid that works there to make the mask for you from vinyl. You need to make a template of the entire surface from masking tape first and then give the person at the sticker shop the template and a picture of the design. It will probably cost you about $20, but worth every penny. Basically you want a sticker of the flames, but when they clean up the sticker before applying the transfer sheet, the actually remove the flames and leave the surrounding material.