One option you've got is to use a pre formed former, talk to WetDecs.
Another option is to screed your own former as a separate operation.
Whichever option you go for you have several choices as to how to proceed.
A generic approach that I would suggest would be:
1) Decide where your drain has to go and shutter off a channel to accomodate it.
2) Pour your concrete slab in the normal way with insulation and DPM underneath, leaving adequate depth for screed. I would leave a good 3 inches bearing in mind you also need to accomodate tiles, adhesive, ditra matting if you need it etc. I usually put the insulation down first onto compacted hardcore blinded with sand or similar, then lay the DPM on top.
3) Your 'screed' will fall into 2 areas. Shower area and rest of floor.
The approach from here on in will differ slightly depending on whether you want to install a pre formed former or screed your own falls in the shower area.
If you are screeding it all yourself then the video will be a good indicator of what you should do in the shower area. However, you will also need to do the rest of the floor. The wetroom I'm doing at the moment has a fall of around 15mm in 1500mm before you get to the shower area. Obviously the datum you'll need to work from is the floor height near the door. Decide your falls and this will give you the drainage height.
In my case I used a pre formed former. The channel left in the concrete slab for the drain worked well in my case as I didn't have to commit myself to the correct position of the drain until the last minute. As I was using SPF for the tray I simply backfilled the channel with SPF after I had fitted the waste and drain etc, then applied more SPF to the area for the former and laid the former down as per the instructions.
If you are doing a screed, now would be the time to work out the position of the drain etc. Install it, back fill the channel with screed and then screed away. Start with the shower area then work back to the rest of the room, making sure you're achieving the required falls (or close as).
Then tank and tile.
This is a process you want to go through several times before you embark on the job. Your builder has only to give you a concrete slab that you specify.
As you will have a channel through part of the slab, assuming the slab will be say 6 inches thick I would reinforce around the slab where the channel is as a minimum.
I suspect your builder may be thinking of A142 mesh for the whole slab, unless it is fairly thick. A142 mesh is about 8 inch squares made out of welded 1/4 inch rod IIRC.
He should know what he's doing here, it's not difficult.
Hope this helps
but assuming you mean the membrane. I want to use schluter ditra/kerdi combination with joint strips with special 2 part adhesive. And i wanted to put it on top of a concrete profiled by myself instead of using aquadeck for example. The purpose of this wet room is for disabled person to use the shower hence the need for no steps.
