I just use 1/48 scale decals and enlarge them 3x in Photoshop to 1/16, then print them out on decal paper using a color printer. I'm a scale guy and thus pretty anal about correct decal sizes. If the markings I'm doing are simple, then I print them out on masking paper, carefully cut them out using a sharp x-acto knife, apply the masks to the tank, and airbrush them.