Hi and welcome,
I am a frequent Marmox user and can highly recc it.
Just follow the installation instructions and you will have a sound installation. use the correct screws/washers, joint it and tile to your hearts content, please be aware if using the Marmox showerlay wet room former that the outlet for the waste/gulley is plastic and is only adjustable in 10mm increments with plastic risers/shims. I wouldn't use this tray again but it is cheap.
Shame you can't still get 2400x1200 mm sheets of marmox.
The Marmox showerstone tray is far better.