Andrew:

As delicately as I can, every once in a while a poster gives away that he's in way over his head by the nature of his question. You have done so. This isn't refinishing Uncle Bob's nightstand where if you screw it up, you can easily get a do-over.

If you're insistent, buy Michael Byrne's book on tile published by the Taunton Press and study it.

OMFG rip all of that out if you want to contribute to the labor. Be sure you understand about asbestos abatement. Or just hire a Pro with a clue. Will NOT be cheap.

You need to remove curb and correct framing first.....

after you frame the new entry properly with ( likely required steel studs and add proper perimiter and cross blocking.......

In youre situation I usually opt for a Thinbed weep divot type shower pan. It will correct the out of level slab, and work with the drain flange As is. pop out those tiles and wet the floor down as you clean the floor off and prime the concrete with a quickrete type bonding primer to encapsulate any cutback adhesive.....after which you can pour a single bonded mud pan and curb and install a complete nobleseal TS or noble valueseal Thinbed shower pan. ( Go to noblecompany website for videos)... -best

If you are going to do a DIY job I recommend buying a pre-cast shower pan and redoing the plumbing so it works with that pan. Dreamline sells good looking composite pans, and Kohler and others sell high quality cast iron shower pans. You can still tile the walls and have a good looking shower, but won't have the complications of installing a floor liner and trying to slope tile, etc.

Bruce

I've used Tile Redi shower pans. You should buy a pan.