I have had to take my old corner unit shower out and about to install a bigger shower. What I'm going back with is Cultured Marble. This will be the pan and the walls. I was told I needed to install the pan and prep the walls with the green board prior to them making and installing my walls. My question is do I install the green board first or the shower pan?
Shower pan is always first because you want the top of the shower pan to the edge of the green board. So there may be shimming involved to get the walls not only lined up with the pan but as close as you can to perpendicular.
I am not going to argue with your installers because I have had some large sheets of granite and marble installed for showers but I would recommend a backer board not green board. Even if they say green board is fine because the water is impervious to their tile that doesn't mean water can't get on the green board from behind or up top. And if they are gluing the sheets down there will eventually be issues. Using Hardiboard would take water coming from any direction out of the equation.
I don't know how far along you are, and I've never installed a shower, but I have installed a few bathtubs from scratch, and I assume the principles are the same. I frame the enclosure to allow 1/4" extra all around, i.e for a 60" long tub I frame the opening 60-1/2" long. I make sure the corners are as square as I can get them, and the walls are straight and plumb. I make sure the studs in the corners are firmly tied together to minimize any flexing if someone heavy leans on the wall, and I make sure that there will be enough stud showing in the corner to fasten the boards on wall #2 after the boards are up on wall #1. I put blocking where any horizontal joints will be, including above where the top of the tub will be. I install the tub, shimming the studs with 1/4" shims to 1/4" above the top of the tub lip. I install 1/2" backer board 1/4" above the lip of the tub. (I agree with DMoore, above, about greenboard, I won't use it for a tub enclosure) Since the framing is 1/4" "outside" the tub, the inside face of the 1/2" backer board is now 1/4" "inside" the tub. The lip of a tub is usually 1/8" to 1/4" thick, so the finish material (tile, marble etc.) can overlap the lip of the tub smoothly, with the back of the finish material barely touching, or almost touching, the inside face of the lip. If your shower pan lip is more than 1/4" thick you might need to adjust accordingly. Also, if you are thinking of going this route, you might want to check with your marble supplier to make sure that this extra 1/2" won't throw off their calculations. Backer board is much more troublesome to cut than greenboard, and it is important to avoid breathing the dust. You can attach it with special screws designed for the purpose, or 1-1/4 galvanized roofing nails. The screws are hard to drive, an impact driver is the way to go, I find. I usually use half screws and half nails, because I prefer screws, but nails are quicker and much cheaper. Fasteners should be 8" OC, and should not be placed within 2" of the corners of the board, nor within 3/8 of the edges. I actually have a scrap of 3/8" plywood 8" long and 2" wide for this purpose.
Your shower pan kit should have instructions on how to install it.
Placing the green board in first, and then trimming it to slide the shower pan in isn't all that unusual.
I'd recommend either buying your shower pan first, so you can read the instructions before you start putting in the greenboard. If you don't have the space to hold the shower pan, select the shower pan and look online for the installation instructions.