Here is a pretty general outline of what needs to be done when making a mold.
1) Clean, buff, polish, wax the part. The mold will literally be a mirror image of your fender, so if you have any scratches and dents they can and will show up. So you want the best finish you can achieve.
2) Use a mold release wax, and put on 2-3 layers, this will aid in removing the mold.
3) Optional, apply a layer of PVA (poly vinyl alcohol), this is a liquid that dries into a thin film, its added protection from the fiberglass sticking, its also self leveling so it will fill in small imperfections.
4) optional, apply a coat of gel coat, this is usually a somewhat thick polyester gel coat, commonly sprayed on using a specialized gel coat gun, but can be brushed on. This layer will allow you to polish and wax the mold and gives a smooth surface to make a part from.
Also take note of the shape of the fender, i believe a two part mold will be needed to make a fender. Imagine taking a large piece of cloth and wrapping it over the fender, now imagine that cloth getting hard, will you be able to remove the fender from it? you need a two part mold so it can be taking apart to allow the fender to be removed without damaging it.
My friend who does quite a bit of composite work taught this to me, so i learned from watching him but never doing it myself.
Alex731 has a build thread on his S30, and he mentioned doing some composite work, so maybe he knows more on the process.