Where the back panel makes contact with the base, apply a bead of  sealant. (Photo F) Replace the back panel onto the base pins and  against the wall. Line up the panel with the marks on the wall studs.  Install screws into the top outside corners of the mounting flange first. Use the holes drilled in step 3.

Do not over tighten the screws and damage the flange!

Wipe out any sealant that is pushed out of the seam with a damp, soapy rag or sponge making sure that no sealant is visible.

 STEP FIVE;

IMPORTANT! If you noted a variance in the vertical seam earlier, now is the time to correct this. Pull straight out on the back panel pins with a pair of pliers, pulling the back panel mid section into position. Install a screw in the mounting flange to hold, and then install the remainder of the back panel mounting screws. Test fit the side wall panels again. (See attachment for drawing)

The side wall panels are installed in the same manner as the back  panel in step 4, except that there must also be an additional bead of sealant placed to the rear of the vertical joint where the side panel  meets the back panel. Also place a loop of sealant around the  elongated slot on the side wall panel edge. Install the panel.

Be sure to wipe out any visible sealant.

Allow all sealed joints to cure for 12 to 24 hours before getting wet.