Cut the riser drain pipe to the correct height by setting the shower drain fitting around the pipe, marking where it intersects the pipe, and then cutting with a hacksaw. Or, if you plan to install a mortar bed, add the thickness of the mortar bed to your measurement, and cut the pipe at this point.
Place a larger 4-inch section of pipe or a 4-inch coupling around the base of the pipe. Place a level over the coupling to ensure it does not rise up above the top of the concrete slab and that it is centered around the drain pipe.
Fill in around the coupling with a mortar or ready-mix concrete and a trowel. Tamp down the top of the fill with a tamping tool to make a flat and smooth surface. Let this dry for 24 hours.
Slide the shower pan over the drain pipe. Connect the shower drain fitting to the drain pipe to test the fit. Apply PVC cement around the drain pipe and drain fitting, connect the two and let dry for four to eight hours or as recommended.