First, wash the body thoroughly in warm water with liquid dishwashing detergent. Use an soft, old toothbrush or similar implement to scrub all resin parts clean of the mold release used in its construction. Mold release will cause paint to rub off of the body; once you think you’re done, clean it again.
The casting flash can be easily removed with a modeling knife. Be careful not to remove any of the edge of the body when cutting the flash. If you use snips to remove flash, wear safety glasses, as the resin is fairly dense.
Trial fit the body to the chassis you wish to mount it on. You can adjust the length of the mounting posts and the size of the wheel wells to get many different effects. Sand or file the mounting posts flat. To drill the body mounting holes, use a 5/64" bit and a pin vise to drill the holes 3/16" deep. Drill the rear hole first. Use the chassis as a guide for drilling the front hole. If you make any mistakes, a good two-part epoxy can be used to refill the hole. Wait until the epoxy dries thoroughly before attempting again. The front post may rub on some axles, so check for interference and sand or file down any spots that rub.
Your body may warp if it gets hot during shipping. This is not a big deal. Place the body in hot water for a few minutes and work out the distortion. This should only take a light effort. If it becomes difficult, reheat the body. Bending the body when it is cool could cause it to crack.
At this time you can clean up the body edges with fine sandpaper or sanding sticks. This is also a good time to do any custom work. If you are a little exuberant or find a bubble under the surface of the plastic, you can mix up decent body putty with talcum powder and some Pacer Slo-Zap CA glue. If you worry about the cumulative effects of talc (like you worry about talc and not CA glue), cornstarch could be used. Other alternatives are two-part epoxies like Plastic Weld or 5 minute Epoxy or a molding putty like squadron putty.
Testors sanding film can be used to put on a smooth finish before painting. Any paint should work, though priming the body is a must. Test your paint under the body first to make sure it will adhere to the body and not rub off. You will also find out how well you cleaned it at this time.
Make sure the windows fit tight. You can use two-part epoxy or CA glue to secure the parts. Make sure to remove any paint from the areas where glue will be applied.
Have fun putting together your kit. This is supposed to be a fun escape from the day to day world. Don't take it to seriously. If you find yourself getting frustrated, take a break and come back to it when you are calm. You Will get more done that way. It can take hours to undo a mistake made while you were tired.