Use plenty of water on the surface (a running garden hose is good) and start sanding the car. Do not press hard on the sandpaper, and when it dulls, discard it and wrap a new piece around your sanding pad. Use only short strokes (no more than about five inches long) so you won't make long scratches if any grit gets under the sandpaper. Use a squeegee periodically to draw off the water on the paint surface, and check your progress. Keep working until all the orange peel and light scratches are gone.
This process can take a day on a full-size truck, so be patient and don't try to rush it. Put some oldies on your boom box and settle in. Never color-sand sharp corners such as the edges of hoods or door skins because there is very little paint on them. If you are at all worried that you might accidentally go across such edges with the sandpaper or a buffer, tape them off with masking tape to protect them. When you have gone over the whole car completely, wash it and tack rag it again to make sure it is clean.
For the next step, you will need a variable speed buffer. DeWalt makes a good one, and so does Makita. But whichever brand you buy, make sure it is a variable speed type. Old pros who know what they are doing can use a high-speed buffer safely, but an amateur will ruin a paint job in a hurry with one.
Pick up a System One polishing kit. It contains the buffing compound, a glaze, three different grades of buffing pads, a microfiber finishing rag, and a CD outlining the whole color-sanding, buffing and polishing process in detail for professional body shop clients. As we said before, this kit is intended primarily for body shop pros doing high-end paintwork, but the company is happy to sell their products to hobbyists, too.
Always buff your car in a well-lit, clean area, whether inside or out. It is important to be able to study a vehicle's finish from a number of angles, and that takes bright, even lighting. Dark-colored vehicles can be a special challenge. Florescent lights are good for finding spider web scratches, but a bright overcast day is excellent, too. Bright sunlight can be a problem, though, because you could be dazzled by the shine.
All you need for most paint is 2,000-grit microfine sandpaper available at automotive pain
Let the paper soak for 30 minutes, then wrap it around a sanding pad.
Keep the surface wet and lightly sand using short strokes to avoid scratches.