Place the fiberglass neo-angle shower floor or base where it will be installed. Use a pencil to draw a line on the drywall along the top and front edge of the base. Move the base out of the way. Measure in 1/2 to 3/4 of an inch from the front edge pencil lines, depending on the thickness of the drywall being removed. Draw the new adjusted line parallel to the original front edge lines.
Cut along the pencil line on the drywall, using a utility knife. Break the pieces of drywall by hand, or with a hammer, below the knife cuts and remove them.
Reset the fiberglass neo-angle shower floor or base. Push it all the way back into place up against the exposed wall studs. Use a pencil to trace the shower floor drain opening onto the floor below. Pick up the base and set it aside again. Cut the drain hole in the floor with a reciprocating saw. Put the drainpipe in and connect it to the main house drainpipe. The top of the drainpipe must be flush and level with the floor below the shower floor.
Set the fiberglass neo-angle shower base on its side to put the drain body into the base drain hole. Use one rubber ring gasket on either side of the fiberglass base and tighten the hex nut on the bottom of the base by hand.
Put the fiberglass shower base in place on the floor against the wall studs. Check it with a level. Remove the base and spread floor-leveling compound on the floor if it is not level. Replace the shower base and check it again with a level.
Affix the filler strip, supplied with the fiberglass neo-angle shower floor, along the top edge of the base, using glue. Place it even with the drywall above the base to create a level surface.
Put the rubber drain seal and drain seal expansion ring together. Lubricate both sides of the rubber drain seal, drainpipe and inside of the drain body. Insert the rubber drain seal into the drain body so that the expansion ring faces up. Place a piece of scrap pipe over the top of the drain seal and drive it all the way into the drain body, using a rubber mallet. Seal around the expander with caulk. Cover the drain body with the drain screen.