Types of Fiberglass
Any type of fiberglass accessory can be made using forms, fiberglass strips and resin. Shower pans are no different. For a shower pan the fiberglass resin should be cured and hardened to a waterproof shell, so use plenty of it. Buy resin that is made for marine applications.
Depending on what the measurements are for your shower pan, you will want to build forms to match. These forms will be used to shape the fiberglass strips. In the case of a shower pan a flat form with curved walls to match the bottom of your shower will do. You will not have to make a sandwich form, the strips of fiberglass can be simply laid over a structure that mimics the bottom of your shower. Use wood, plastic or foam to make these forms for the best results.
Laying the Fiberglass
When the form is completed lay the fiberglass strips over it and then coat with resin. This sounds simple and it is but there is a hidden art involved. The fiberglass needs to be laid in a crisscross manner, so it overlaps, but it can't be too thick or the resin will not penetrate the fiberglass. Try to find a balance and lay the strips in such a way that they provide a fine mesh that absorbs all the resin.
In some cases you may want to have a professional create your shower pan. Many auto body shops do other types of fiberglass work, and some companies specialize in building custom fiberglass enclosures. The work won't be cheap, so you are probably better off buying a shower pan made at a bath store or home products store.