I'm redoing my master shower after discovering a bad leak. I have no experience with this, so I've been doing a lot of research online. Here's what I have planned. If there's a better way to do anything I'd appreciate any suggestions or advice.

I just replaced the drain. Already replaced all the framing that was rotted out.

1) rubber liner on floor
2) metal lath
3) mortar sloping down to drain
4) cement board on walls
5) seal gaps in cement board
6) install tile

Current picture:

When I demoed the shower, the previous install had a layer of plastic sheeting over the framing. They used greenboard though underneath the tile. Is the plastic necessary?

Treated wood for the curb and any bottom plate

Hardi is not waterproof. Look into red guard. Or Ditra system
I prefer a moisture barrier behind the hardi
15 lb Roofing felt at the minimum run horizontal with the inside corners overlapped 16 inches

My tile guy put 1/4" green board on the walls at 7" high to the deiling. Then the pan could be laid in and tucked in the corners and then covered with rock board. That way you don't have any bulges in the corners where the rubber pan is folded over.

He then sealed the rock seams and screw holes with NP1

For a better system, skip the old method and try a Kerdi waterproofing system. The way that you mentioned, while the acceptable method, allows for too many possibilities for leaks later, or water that leaks through the grout or cracks later that develops in the corners to migrate into the liner area and just sit for years until it leaks into the house somewhere.

Kerdi puts the waterproofing between the tile and the wall or pan surface. And the liner ties back into the drain under the tile so any water that could get behind the tile has a way back to the drain and out of the shower area. It's a little more up front but cheaper than redoing the shower again.

I looked at a 6 year old shower last week that's already leaking into the bathroom and along the foundation outside when they shower. I'm not saying they all leak, but there are better methods for waterproofing now.

https://www.schluter.com/schluter-us/en_US/Shower-System/c/SS

What he said.

I'm doing a commercial project and the architect wants the Kerdi system in the locker room showers. (Not sure why as I've never had issues with a roll on rubberized barrier)

It's taking the way commercial has been done previously and puts it in a residential application.

You have the right idea for the pan but are missing a step between #5 and #6. Use redguard or kerdi or similar for waterproofing walls. I've used redguard in the past and it is easy to apply (see YouTube).

If you use a waterproofing method, don't use plastic sheathing. Don't know how true it is but I read up on something called a "mold sandwich" if you use both methods, back when I was researching for my shower project.

Third vote for Kerdi. You'll pay a few extra bucks but for the DIYer it's more than worth it in my opinion. With your shower size & center drain you might even be able to use their 3x6' presloped foam pan. http://texags.com/forums/61/topics/2754456/2

Here's my remodel from 2015... I didn't use Kerdi as my small town store wasn't well stocked with it. I used GoofProof system for the floor and pan and redguard over the walls. Never had any issues with it over the next 2.5 years.

Sold the house and moved to Houston last fall.

https://texags.com/forums/61/topics/2648316

Realized I never updated that thread after tile...

I can use plain old drywall with the Kerdi, right? Do I need to mud the gaps and over the screws, or just put the Kerdi right on top of it?

You CAN but I wouldn't recommend it. I would use a concrete backer. Not so much for moisture barrier (it will ultimately fail like sheetrock) but for rigidity. If someone were to slip in the shower and push forcefully against a sheetrock wall, it could give way. Not so much with the concrete. There are probably other good reasons that I can't think of now as well. You'll have a shower that lasts as long as the house does.

You can. 91 brings up a good point, but I'm not sure how valid it is since I assume you'll be tiling over it all anyway.

Schluter approves installing Kerdi over drywall and I've done it a few times without any trouble. Also installed it on top of cement board, do whichever you're more comfortable with.

The last shower I demoed out was done like the kind you were going to build. The bottom 3' of the wall showed signs of moisture intrusion (mildewed insulation, black wood but not rotten). It was working out really well for the tiny ants in the wall before I showed up with a demo hammer.

That pre-sloped base looks like it would be way easier. My shower is 36" x 78.5". Would it still be easier if I get the 48x72 Kerdi base, cut down the short side more, and add mortar to the long side? I don't see a 36x72 size, but maybe I'm just missing it?

Do you use the Kerdi membrane on the ceiling as well, or just walls and floor?

Yah they make a 48x72". https://sccpublic.s3-external-1.amazonaws.com/sys-master/images/hc2/h25/8840809054238/Prefabricated_Substrates.pdf

My shower was ~ 34x78". I just cut the edges off and extended mine on the one end (drain was slightly offset).

I didn't use kerdi on ceiling, just floor and walls.

Just ordered all the Kerdi stuff online. Do you recommend the Kerdi brand joint sealer as well? It's $25 per 9.8 oz tube at Lowes, but if it's the best then I'll pay up for it.

I'm not sure what you're even referring to. So either it's not needed or I screwed up.