Clean the surface area around the hole. Trim any glass fibers and and the area with a medium grit paper. Scrub off any dried soap, oil or other products with acetone leaving a slightly rough surface ready for repair.
Measure the hole to determine if fiberglass cloth will be needed to reinforce the repair. Holes or cracks more than 1/4 inch wide will need fiberglass cloth repairs. Larger holes may require addition layers. Very large holes my require an expanding foam product to fill the cavity beneath the tub.