Use a shovel to dig the pond to your desired depth and shape. Pack the loose dirt on the bottom and sides of the pond using the shovel and your feet. Lightly spray the dirt with a garden hose to hold the dirt in place and give it a solid base. Let the dirt dry.
Cover the entire surface of the hole using flexible pond liner. Use heavy-duty scissors to cut the liner to size. Cut the liner big to leave room to weight down the liner for the fiberglass application.
Lay the edge of the liner flat on the level ground and place 5-lb. weights on top of the liner all the way around the perimeter of the pond. Use as many as necessary to secure the liner in place.
Cut a layer of fiberglass mat that completely covers the surface of the pond using the scissors. Cut as many seams as necessary to keep the mat from bunching up. Use the first layer of mat as a pattern and cut five additional layers of mat the same size.
Add the catalyst to a bucket of resin following the recommendations on the containers. Mix the catalyzed resin with a stir stick for one minute. Fiberglass uses methyl ethyl ketone peroxide as catalyst; it chemically reacts with the resin and causes it to harden.
Wet the surface of the liner with the resin mixture using a felt roller. Apply the first complete layer of mat and saturate it with resin using the felt roller. When the entire layer is wet, remove all the trapped air bubbles using an air roller. Follow this process for each of the next five layers of mat and let the fiberglass harden.
Put on a respirator and grind the fiberglass so it's level with the ground surrounding the pond using an electric sander with a coarse-grit sanding disk. Sand the surface of the fiberglass smooth using 100-grit sandpaper on a palm sander. Clean the dust out of the pond using a broom and dust pan.
Prepare the surface of the fiberglass for gel coat by wiping it clean with rags and acetone. Let the acetone dry.
Add catalyst to a small bucket of gel coat following the recommendations on the container. Thoroughly mix the gel coat and catalyst together using a stir stick.
Apply the gel coat heavily on the fiberglass using a felt roller. Let the first coat harden and apply a second heavy coat. Let the gel coat harden for 24 hours before filling the pond with water.