Begin by preparing the shower alcove. If this is a replacement, carefully remove the old shower walls and base. Disconnect the old drain from the waste pipe. Thoroughly inspect the subfloor and wall studs for any signs of water damage, rot, or mold. Any damaged wood must be repaired or replaced before proceeding to ensure a solid foundation. Clean the area of all dust and debris using a shop vacuum.

With the shower pan turned upside down on a protected surface (like cardboard), install the drain assembly. Follow the manufacturer's instructions, which typically specify using either plumber's putty or 100% silicone caulk. Apply a rope of plumber's putty or a bead of silicone around the pan's drain opening. Insert the top drain flange from inside the pan, pressing it firmly into the sealant. From the underside, place the rubber gasket and then the bottom flange (or compression nut). Tighten the assembly using channel-lock pliers or a dedicated drain wrench until snug. Clean away any excess putty or silicone that squeezes out.

Carefully place the shower pan into the alcove. Push it tight against the studs. Place a 4-foot level across the top edges (thresholds) of the pan. Check for level side-to-side and front-to-back. The pan's floor is pre-sloped, but the thresholds must be perfectly level. If the subfloor is uneven, you may need to use shims under the pan's feet to temporarily achieve level during this dry fit. This helps you understand how much mortar will be needed. Use a pencil to trace the location of the drain opening onto the subfloor and mark the height of the pan's top flange on each stud.

Remove the pan from the alcove. Based on the drain location you marked, ensure the drain pipe in the floor is positioned correctly. You will likely need to cut the waste pipe to the correct height. For a solvent-weld drain, the top of the pipe should typically be about 1/2 inch below the top of the subfloor, allowing the drain assembly to slide over it. Use a PVC/ABS pipe cutter or a hacksaw for a clean cut. After cutting, place a piece of tape over the pipe opening to prevent mortar and debris from falling in. Give the subfloor one final cleaning with a shop vacuum.

In a 5-gallon bucket, mix the mortar with water using a mixing paddle attached to a drill. Follow the manufacturer's ratio. Aim for a thick, stiff consistency, similar to packed sand or peanut butter. It should hold its shape and not be runny. Using a trowel, spread a layer of mortar 1 to 2 inches thick across the subfloor area where the pan will sit. Distribute it evenly in mounds, but leave a small open area immediately around the drain pipe to avoid getting mortar in the connection. Ensure the entire bottom of the pan will be supported.

Remove the tape from the drain pipe. Carefully lower the shower pan onto the fresh mortar bed, guiding the drain fitting over the waste pipe. Press down firmly and evenly across the entire floor of the pan. You can walk in the pan (with clean shoes) to fully bed it into the mortar. Immediately place your 4-foot level on the pan's thresholds again. Check for level in all directions. Gently press down on any high spots to push the pan further into the mortar until it is perfectly level. The mortar should support the pan, and it should not rock.

Once the pan is perfectly level, permanently connect the drain. For the assumed solvent-weld PVC drain, work quickly. Use a dauber to apply PVC primer to the outside of the waste pipe and the inside of the drain assembly fitting. Immediately follow with a liberal coat of PVC cement on both surfaces. Firmly push the pan's drain down onto the waste pipe, giving it a slight twist to spread the cement. Hold it in place for 30 seconds to prevent it from pushing back up. Clean any excess cement with a rag.

With the pan level and the drain connected, secure the vertical mounting flanges to the wall studs. Drill pilot holes through the pre-marked spots on the flange directly into the center of each wall stud. Fasten the flange using 1-1/4 inch galvanized roofing nails or pan-head screws with large washers. The large head of the nail or washer prevents it from pulling through the plastic flange. Install one fastener per stud. Do not overtighten, as this can stress and crack the flange.

The installation is mechanically complete, but the mortar must fully cure to provide its final strength. Do not stand in, place tools in, or otherwise disturb the shower pan for at least 24 to 48 hours. Refer to the mortar bag for specific curing times. After the curing period, carefully step into the pan and walk around. It should feel completely solid and have zero flex or movement underfoot. This solid feel confirms a successful installation.