Line the bottom of the shower floor with felt paper cut to fit the floor and go up each wall 7 or 8 inches. The felt helps contain the mortar, especially as it is applied near the walls, which are often only studs when showers are being constructed. Staple the felt paper to the floor and the wall studs. Fit the material tightly to the corners where the wall and floor meet. Cut the opening for the drain and plug the opening with old rags so mortar does not fall into the hole.
Calculate the slope needed for your shower. Mark the height to which the mortar will need to be built along the walls, using a carpenter's level as a straightedge. The level will also help keep the line level all the way around the shower compartment.
Mix a batch of mortar in a bucket. Stir in the water but keep the mortar on the dry side, something close to the consistency of a stiff mud. The drier consistency will help the mortar retain its shape. A wetter mix will tend to level out rather than form a slope.
Trowel some mortar onto the felt paper, about a quarter inch thick. Embed wire mesh into the mortar you just troweled to add strength to the application.
Apply more mortar with the trowel, thicker toward the wall, thinner toward the drain. Create the slope by working the mortar with your trowel. Smooth the material from the wall toward the drain, keeping the mortar up to the line at the walls and around a half inch thick at the drain.