Above The Shower Pan Membrane, Install The Wall Liner
The next step after the shower pan membrane is in place is the wall covering. The current practice is to build the wall covering with cement board. Here's how...
See, cement board will soak up water, but it won't rot and come apart when it's wet. There are several brands of cement board. There's Wonderboard and Hardi-Backer along with others. They are much alike.
It's not as simple as just nailing up cement board.
Behind the cement board you want a layer of waterproofing material. That's usually a layer of 15 pound roofing felt or 4 mil poly sheet. Just staple to studs. Don't skip the waterproofing layer.
Now about the framing...
The better the framing job, the easier it is to get an acceptable final tile installation. You really may need some shims and adjustment to get the right surface for laying tile.
Here's the other thing about cement board. Get board that's stiff enough. That means product that's only 1/4 inch thick may be too springy. Check with your supplier.
Put the cement board over the felt or plastic layer using galvanized screws to avoid rust. Leave a gap between boards of about 1/4 inch so you can tape and mud all joints. Use mesh tape and thinset for finishing all joints.
Key Point...
Only install the wall boards after the liner membrane is in place.
Here's another thing. Do not rest the wall board on the liner board when you install it. Know why? The wall board will soak up water from the liner and begin to be a mold bed.
Raise the wall boards above the shower pan liner at least 1/2 inch. Better yet, raise the wall boards up to just the level of the second mortar layer so it's completely out of the shower pan and less likely to wick up moisture. An even better idea... Coat the bottom few inches of the cement wall boards with a sealer like RedGuard front and back before installing. That way no water can wick into the cement board.
The standard way to install the cement board is to set the bottom edge about 1/2 inch above the liner membrane. Then the top mortar layer is run up the cement board at whatever level is required to get the right slope.
Remember to only put fasteners in the cement board up near the very top of the liner membrane.
Either side of the cement board is OK for tiling over. The smooth side can go in or out.
Now, for the second tile shower pan mud layer...