You'd only want to use something like tar paper if you were building this on a wooden subfloor. On a slab, you want to bond it to the slab. A couple of ways to do this, either mix up some Portland cement into a slurry and spread it all over, then before it starts to set up, put down enough of your deckmud to cover it. Or, you can cover the area with thinset and do the same thing. Do not use the greenboard on the walls.
There are all sorts of little gotchas when building a shower and you need to educate yourself a bit more. Things like use of blocking at the lower edges of the wall to support the liner, notching the studs in the corners to prevent bowing out the board because of the thickness when you make the folds, use of the corners on the curb, and the list goes on and on. It's not hard, but it is VERY detail oriented - miss one step, and you can have problems.
Setting the slope isn't all that tough. Once you get the drain installed, you measure to the furthest corner, then calculate how high it needs to be with a 1/4" per foot. THen draw a straight/level line all around the perimeter of the shower. Pack a rim of deckmud so it tops out at that line all around the shower, then using a straightedge, make a nice even slope to the drain with the rest of the deckmud.
There are other methods to construct a shower and the one I like is Kerdi from www.schluter.com. You can either use their preformed pan (costs more) or build your own out of deckmud, just like you would for the conventional liner. But, you then install the membrane and tile directly to it...you don't need the second layer of deckmud on top of the preslope and liner. Less chance of messing up one of those steps, and you end up with a shower that has its waterproofing immediately beneath the tile rather than buried over an inch below. FWIW, the tile is NOT the waterproofing layer in a showerpan, it is the liner. No matter what you do, some moisture will get beneath the tile, percolate through the deckmud, then drain down the liner to the weepholes. With the Kerdi system, there are no weepholes because the liner is bonded directly to the drain. THey have some videos on their website so you can see what I'm talking about. Also, you might check out www.johnbridge.com and look in their 'Liberry', which shows how to build a conventional shower (which is what you were describing), but also has help in their forum on that type and other tested/approved methods to build one.
The industry standard is the TCNA handbook, which lists all of the approved methods to build a shower (and other tiling issues).