Remove the tub. Knock out the drywall above the existing tub walls with a hammer, and pry the old shower walls from the framing with a chisel and pry bar. Cut the tub walls with a reciprocating saw to assist in removal. Cut the walls in half and pull them from the tub area.
Cut the drain pipe underneath the tub with a reciprocating saw to remove the tub.
Cut the caulking around the tub with a razor; unscrew the drain with a drain wrench and pull the tub out of the bathroom. Cutting the tub in half assists in removal. Clean the shower area and remove all debris with a broom.
Shower Stall Installation
Level the floor by installing wood shims on any areas that fall beneath the level line. Use a carpenter's level to test the floor for levelness. Screw the wood shims to the floorboards with 1-inch screws and a power screwdriver.
Install the shower stall base in the showering area and push it right up against the wall frames. Look for gaps along the edges of the base, and install furring strips with 1-inch screws on the wall frames to fill these gaps. Check the shower stall base for levelness with a carpenter's level.
Trace around the drain hole to mark a location for the shower stall base with a pencil. Remove the base and cut this new drain hole into the floor with a jigsaw. If the old drain hole lines up with the shower-base drain hole, this step is unnecessary.
Install the shower-base drain. Slide the top half of the drain through the drain hole. Slip the gasket around the bottom of the top half of the drain. Connect the bottom half of the drain to the top half and tighten it by hand. Tighten the drain with a drain wrench to secure it in the shower base.
Place the shower stall base back into the shower area. Fit the drain into the drain hole. Check the base with a level again and mount the base to the wall with the provided mounting screws and a screwdriver. Drill pilot holes for the screws with a power drill and 3/16-inch bit prior to mounting. Install shower stall walls on the wall frames. Mount each wall to the framing with the provided mounting screws after fitting the walls over the lip of the shower-stall base.
Connecting the Drain
Fit a new drain trap to the existing drain connection in the plumbing below the shower stall. Fit PVC pipe fittings between the shower stall drain and the main drain line. Make sure the pipe runs from the drain hole to the main drain line.
Remove pipe fittings and coat each with PVC adhesive. Reconnect the pipe fittings to install the drain. The drain pipe will extend up into the shower area through the drain hole in the floor.
Slide the new drain pipe through the drain hole. Lubricate a compression gasket with plumber's lubricant and fit it over the pipe. Tap the compression fitting with a hammer and block of wood to compress it onto the shower drain and seal the connection.