Lay out the first sheet of Wonderflex you want to use. Use the scissors to cut out the shape you want to add to your fiberglass costume. Keep any scraps or extra pieces. You can use them later to add small details or make repairs if you make a mistake.
Place the sheet of Wonderflex into the mold you want to use. If you plan on molding by hand, you will need heat resistant gloves to mold the Wonderflex as it will be extremely hot. Use the hot air gun to heat up the material until it reaches a temperature of 150 to 170 degrees. You can also use an oven if you do not have access to the hot air gun.
Begin molding the material as soon as it reaches the desired temperature. Wonderflex will be moldable for three minutes after you remove it from the heat source. If it is in a mold, press the material to shape it, or use your gloved hands if you are not using a mold. Allow the material to dry before removing it from the mold.
Place the molded material on the fiberglass at the location you want to add it. Use the hot glue gun to glue the edges of the mold, then attach it to the costume. Let the hot glue dry for several minutes. Once it is dry, you can use paint to add color to the Wonderflex.