A while back, I bought a 1974 chevy k-5 blazer body for my rock crawler. It was cheap, like $20 on eBay, but it isn't exactly what you would call scale worthy. Its just a shell, so I added things to make my body more like the pro guys have such as fender wells and lights.
Step 1: Inner Fender Wells
1) Use a wire to make the shape of the fender and use this as a guide for making the actual fenders. Write down the length of the wire
2) Decide how deep you want the fenders to go into the body and cut a strip of coroplast that width and the length that the wire was.
3) Cut through one layer of the coroplast to bend it in the spots where the wire is bent. This should make the coroplast have the same curve that your fender does.
4) Glue two up-right pieces in the body so they are the distance to the edge of the body is the same distance that the fender well is wide.
5) Glue in the fender wells. You can run a bead of glue along the inside seem then lick your finger * and push it in the crack to get the glue in deep. This also makes it look better.
6) Make covers for any other areas you want by cutting through one layer of the coroplast allowing it to bend to the shape you want.
7) Make a bunch of supports and glue them in.
* Just warning you, if your finger is not licked, you WILL get BURNED.