To install a shower unit, remove the faucets, escutcheons, shower head, tub spout and drywall from the area surrounding the existing bath, and disconnect the plumbing after turning the water source off. Mark the drain hole, and cut along the mark through the subfloor to form the drain. Drill through the appropriate joists to form a path for the drain. Connect the drain and vent lines, and close the floor. Connect the water supply lines, and set shower into place.
To remove the drywall easily, cut through the wall with a utility knife, and pull the pieces of wall away from the frame in large pieces. When creating a drain, slope the horizontal lines by 1/4 inch every foot, and use a 2-inch drill bit for 1 1/2 inch drain lines.
Install a P-trap directly below the drain hole, and set the plumbing into place. If you need a reducer to transition from 2-inch to 1 1/2-inch pipe, install the reducer directly below the shower.
After running the drain lines and connecting a vent, use plywood to close the floor, and reconnect the water supply lines if necessary. Set the new shower base into place, and secure it with mortar, adhesive or screws. Follow the installation instructions for the shower to complete the drain hookup. Allow all plumbing glue, adhesive or mortar to dry completely before using the shower.