View photos or video of the item from which you will model your replica. If possible, inspect the original.
Build an inner form for your object out of wood, chicken wire, foam, newspaper or tape. This must be the size and shape of the object that you are recreating, but the specific materials and method of construction are not as important. Keep the form hollow and lightweight so that the finished piece will not be too heavy.
Cover the form with a "skin" of plastic wrap, cloth or plaster bandages. This will create a uniform surface for the fiberglass.
Tear the fiberglass mat into squares. Five-inch squares are an easily manageable size for most projects, but you might need bigger or smaller pieces, depending on your project. If you're making a fiberglass elephant, for example, use larger squares.
Mix a batch of fiberglass resin and catalyst in a disposable bucket. Follow the instructions on the product to attain the correct ratio.
Paint a layer of catalyzed resin onto the form. If the form is large, work in sections.
Add a layer of fiberglass squares onto the form. Overlap them slightly and press them into the resin.
Brush more resin onto the fiberglass. Saturate it fully and work out any trapped air bubbles by tapping them lightly with the brush until they disappear.
Add another two to five layers of fiberglass and resin. How many you add will depend on the size of the object you are creating.
Allow the fiberglass to fully cure for several hours until is completely hardens.
Trim off excess fiberglass with a rotary tool.
Sand the fiberglass until it is smooth.
Paint the object to match the original work you are depicting. If desired, seal it with a few coats of clear varnish.