Measure the width and depth of the cast-iron shower base with a measuring tape. Measure the shower enclosure frame to ensure that the base will fit in the enclosure. Also, measure the height of the lip along the perimeter of the shower base.
Place the cast-iron shower base inside the shower stall fame to ensure that the drain hole in the base lines up with the drain installed through the subfloor.
Cut a section of 5/8-inch-thick plywood to the width and depth of the cast-iron shower base with a table saw.
Rip four sections of 1-by-4-inch lumber stock to the height of the shower base lip with a table saw. Cut two of the sections to the width of the shower pan and the remaining two sections to the depth with a circular saw. Miter the ends of the sections at 45-degree angles with a miter saw.
Place the lumber stock sections along the perimeter of the plywood with the outer edges even with the plywood edges. Drive 1 1/2-inch wood screws through the lumber stock sections into the plywood at 4-inch intervals using a drill equipped with a screwdriver bit.
Place the cast-iron shower base on the plywood frame. Draw an outline along the drain hole with a pencil. Remove the shower base and drill a 1/2-inch hole inside the drain hole outline so that the edge of the hole meets the outline. Insert the blade of a reciprocating saw into the drill hole and cut along the outline.
Locate the floor joists beneath the subfloor with a stud finder. Mark the locations of the floor joists inside the shower stall area on the plywood, and snap a chalk line between the marks.
Place the plywood frame inside the shower stall frame. Drive 4-inch wood screws through the plywood into the subfloor and floor joists at 4-inch intervals.
Cover the plywood frame with waterproof membrane, pressing it into the corners between the plywood and the lumber stock sections along the perimeter. Staple the membrane to the plywood frame with an electric stapler. Trim excess membrane with a utility knife.
Place the cast-iron shower base on top of the plywood frame and waterproof membrane. Insert the drain assembly through the drain hole and into the drainpipe.