Dee, The tools used for prepping flats are the same tools used for prepping resin and metal rounds - hobby knives, sanding sticks, fine files, etc. You can use the same putties/fillers as well: Squadron Putty, Milliput, Aves Apoxie Sculpt, Kneadatite (Green Stuff), etc. Ditto for primers and paints. If you're already well into rounds, you're pretty much set as far as tools and materials go.
Painting-wise, rounds are generally painted with the 'halo of light' overhead. Flats can be as well, but often there is some specific directionality. I tend to use a light source coming from the front upper left. With that in mind, you have to look for body parts and whatnot that might cast a shadow over another part of the piece. It'll all depend on the figure's pose.
Good luck with it.
Glen