Measure the old shower pan and take measurements from the edge to determine where the drain pipe is located. When you purchase a new shower pan, be sure it fits with these measurements.
Set the new shower pan in place and use a level to be sure it's even. If there's a higher side, nail 1/2-inch plywood strips to the subfloor to create an even base.
Place the strainer inside the drain hole of the new shower pan from the top. Keep the gasket over the strainer to make it watertight. Put another gasket on the other side and secure the strainer from the bottom of the shower pan with a nut. Tighten the nut using a wrench.
Put the new shower pan back in place with the strainer lining up with the existing drain pipe.
Drill pilot holes along the shower pan where the studs are located in the wall. You may have to use an electronic stud finder to locate them. Once the pilot holes are drilled, screw the shower pan to the studs with rust-resistant screws.
Connect the strainer to the PVC drain pipe by pushing a compression gasket over the drain pipe. Tap the gasket with a rubber mallet to force it into place. Then, put the top strainer piece on to complete the connection.