Decide whether you want to change the colour of the car completely or just want to get a new look for your doors or some other particular part of your vehicle. If you want to get a new paint job for entire car, your workload will be a lot greater.
Inspect the surface of the car and mark any corroded or worn out patches on the body of the car, they will require special attention. After marking all damaged zones with a marker, use a bondo or a grinder to remove rust and other form of debris from the surface of the car. However this will only work for small patches, larger areas will require new metal pieces or hardware clothing to cover the patch from underneath.
On completing the repair work spray them with a primer or some other similar form of agent. Purchase the required amount of silica from a hardware store depending on the surface area of the car and put it in the sand gun.
Turn on the sand gun and allow the pressure to build up to 100 psi. Wash the car with water and give it ample time to dry out. Wipe the surface of the car with an emery cloth, dip the cloth in thinner and rub it rigorously on the surface of the car. It's purpose is to remove any dust which may be still present, and removing some part of the old paint is a bonus.
Wear your least favourite clothes and don your protective goggles and gloves. Use the sand gun to spray silica over the surface of the car at high pressure, the nozzle of the gun should not be brought closer than 6 inches or it will damage the surface of the your vehicle permanently.