Fibreglass is a good material to create vanity car parts like designer bumpers. The sheets can be moulded into the shape you want and you can then create that shape by mixing with a polyester resin. In order to properly mould a fibreglass bumper, you must first create a mould that will help you shape the fibreglass sheets into the size and shape you need. You also must make sure the fibreglass itself won't bond to the mould.
Create the mould--also called the plug--for your custom bumper, using the original bumper for a guide with whatever materials work for you. You can carve blocks of foam into the shape you want, or you can place cardboard around the old bumper. Make sure the plug will fit the car, then add whatever customisations you want, like spoilers or holes for lights.
Apply a polyester primer to the plug's surface, then sand down the primer with 180 grit sandpaper until the primer is even. Polish the surface to a glossy finish and then let it sit for a day or two. Wax the bumper with a special mould-making wax, wait for it to dry and wipe away excess wax with a clean cloth; repeat the waxing 2 to 3 times.
Prepare the mould for the fibreglass by applying an aerosol gelcoat to the surface; the thicker the gelcoat, the better. Apply three coats to the surface and wait for it to get tacky. While you wait, mix the polyester resin as its instructions tell you.
Apply one coat of the resin to the bumper's surface with a brush, then lay the fibreglass sheets onto the surface while the resin is wet. It can help to cut the sheets into smaller pieces beforehand so they will lay onto the surface easier without large creases or other uneven surface problems. Smooth away any air holes with the brush or a roller.
Add more layers of fibreglass by applying another coat of resin, making sure the previous fibreglass sheet is soaked, and lay the next sheet on top. Lay 3 to 6 fibreglass sheets on the mould this way. Apply one final coat of resin to the last sheet and wait for the resin to dry and harden.
Pry the finished bumper off of its mould/plug. Use a flat, strong object like a mixing stick to help gently pry it off. Blowing air in between the bumper and mould can also help.
There are two different kinds of fibreglass: mat and cloth. Cloth is generally the stronger of the two, but mat can leave a smoother finish because the glass is woven instead of laid out randomly.