Clean the floor and the walls in the area with an all-purpose cleaner where you're building your shower pan. Do this as thoroughly as possible, leaving behind no residue and soaking up as much excess moisture as you can.
Measure the distance from the drain to the walls to determine how much of a slope your floor will need. The general rule of thumb is about 1/4-inch slope for every foot of distance to the drain. Therefore, a wall that sits 3 feet from the drain will need to be 3/4 inch higher than the drain itself.
Determine the entire area of your shower based on your initial measurements. Cut pieces off 15-pound roofing felt to lay across the floor. Lay pieces to overlap one another. These pieces will keep your concrete from drying out faster than it should.
Fasten galvanized mesh over the overlapping pieces of roofing felt. Secure these in place by using steel galvanized screws that you can drill directly down into the wood subfloor.
Combine three parts sand with one part of Portland Cement to make mortar.
Mark a spot on the floor where your shower will end. This spot should be located about 1 foot beyond where the water will hit the floor. This spot also points out where you later add a 1/2-inch rise to the floor to prevent the water from escaping the shower.
Mark the spot on the wall where the mortar should rise, once you determine the rise for the entire shower. Mark each wall with a pencil line indicating where the shower pan will rise to.
Spread the mortar with a trowel on the floor, beginning on the outside until you build up enough mortar to reach the rise line. Apply a small thin layer around the drain also. Continue to spread from the walls toward the drain in a downward fashion. Take care with each spread of the mortar that you are creating a gradual downward slope toward the drain. Be sure to create the extra 1/2-inch rise on the side of the shower that will be shower entry.
Place the drain assembly inside the drain hole and on top of the thin mortar layer. Allow the mortar to dry overnight.
Apply a thin layer of thinset along with a waterproof fabric membrane on top of the mortar once it has dried. Spread out the thin membrane with a V-notched trowel. Allow these to dry before proceeding to complete the walls and the shower fixture.