Once the pan is installed, starting with the back wall, put a bead of the sealant along the step joint of the pan.
Stand the first panel in place with the step joints overlapping.
Put 1 screw with a washer at the joint of the pan and wall panel on each stud.
Put 2 more screws with washers up each stud, about 10” apart, leaving the top one for now.
Next, put a bead of sealant along the step joint of the panel you just installed.
Stand the next panel in place and screw to studs the same as the lower panel.
Repeat for the third panel (you will need to cut this panel to the height of ceiling)
You should have 10 screws with washers vertical on each stud.
Once the back is finished, put a big bead of sealant up each corner.
Now you install the side panels in the same manner as the back wall, As you set the side panels in place, you would press the panel into the bead of sealant in the corners.
Once all the wall panels are in place, Cover each screw and washer with sealant.
Smooth out the sealant on the joints and screws with a trowel.
If using other tile backer board, waterproof as below.