A fiberglass shower pan is a very simple thing to build. You'll need to obtain wood or foam to form a base in which the fiberglass strips and resin will be poured into. You can express some creativity with this task by adding decorative elements. The basic steps are straightforward.
Create a Form for the Fiberglass
Use lumber or foam pieces to form a box in the dimensions of the shower area. You will need to connect the ends of the boards using fasteners such as nails or screws. Doing so will give you an area to work with to create the fiberglass pan.
Pour the Fiberglass
Layer the fiberglass strips in the form to form a criss-cross woven pattern. Pour the resin over this pattern and allow time for it to set up and harden. Once it has hardened, separate the ends of the wood form to reveal the fiberglass pan. It can be placed in the shower.