New house - bathroom floor, mixed advice

[Geed]

Hi

Going to be helping with tiling the upstairs bathroom floor in a new house. Just bare flooring right now.

I've read / watched just about every bit of advice from just using PVA to seal the boards first and then tiling directly on top with a flexible adhesive to putting down ply then backer board then matting then the tile. All this different advice coming from tile shops and "pro" YouTube videos.

What is the proper way to do this?

Thank you for all your help

 

[Boxtiling]

Hi there use plastic ply and Mapei s2