Wall Prep before tiling

[Simplez]

I need to prep 3 different surfaces and I am not sure how to proceed.

1 - green moisture board

2 - green moisture board that has been skimmed over

3 - red brick masonry which tile backer board will be dot / dabbed onto

Fitting a bath - so where the wet area is - will be fitting the tile backer board.

However I need to prep the red masonry first - as I am going to use s1 tile adhesive to dot dab the tile backer boards onto the masonry then use mechanical fixings into the dot/dabs. I need to make sure the s1 adesive bonds to the red masonry properly - so how do I primer / seal it?

Do I just tile straight onto the green moistureboard or do I seal that as well first?

What do I do with the skimmed over moistureboard?

 

[RandomGrinch]

I'm not an expert, so please disregard!
Have you considered a tanking kit?
https://www.screwfix.com/p/mapei-shower-waterproofing-kit/78484
It might be completely OTT, but if your room is small it will provide a good homogeneous waterproof layer over multiple surfaces. Especially useful if your shower is over the bath.