Sanding a car is a lot of work. A whole lot of work. And not all of that work actually involves sandpaper. A lot of the work involves prepping the car for sanding. No one likes prep work, but if you are going to sand your own car then the prep work is vital. Unless you have help and can devote several hours straight to this project, expect sanding your car to take several days (or several weekends, depending on your schedule). The advantage to sanding your own car is in the knowledge that you have done the best job possible as well as the satisfaction of doing it yourself.
Remove the headlights and tail lights off the car. Remove any running lights. Trim may be removed if it is practical and not too difficult.
Tape all remaining trim, any emblems on the car, and the rubber around doors and windows.
Use 320 grit (fine grit) sanding blocks available at most paint and hardware stores to hand sand all painted areas of your car. Work carefully around trim pieces, chrome, and rubber so that you do not sand them. Sand off all the clear coat and old paint. It is vital that all the old clear coat and all shiny spots are removed completely from the body of your car as your new primer and paint will not adhere properly to any areas which still contain any clear coat.
Use 320 grit sand paper for small areas that a sanding block can't reach. Take your time and be thorough. Check over the entire car a second or even a third time for any spots that you may have missed or overlooked.
Sand the entire car a second time using Scotch Brite Scouring Pads. Try to sand in the same direction as much as possible. Sand with the Scotch Brite Pads until the car has a uniform look to it. You are now ready to wipe your car down with tack cloths and spray on the first coat of primer.
It is very easy to miss small nooks and crannies when sanding. This is one reason for going over your car several times, looking for any spots, no matter how small, that you may have missed or overlooked.