How a bit of groundwork?? those pictures are sort of fake because this very version pictured here currently lies down the drain in the garden. I had to do all this all over again because of a basic failure I did.
(more on that later). I still post them because I did exactly the same thing a few weeks later.
Anyway here is first a plastic sheet with all the tiles glued, then this plaque put into place on a thick plastic base. then some plaster to add some relief and details.
At this stage you may notice that the level stops abruptly at roughly 6mm high.. All the extra relief will be added afterwards.