It should not move.
How to stop slipping of large tiles on the wall.
Lay out one course of your tile the width of the back wall first on the floor with your chosen spacing.
However i have used blue painter tape to temporarily hold tiles in place.
Don t start it at the floor.
Mark the top of the tile and then using your level transfer that mark all around the shower.
A batten board helps you start your bottom most course or row of tiles.
To achieve an adequate bond for large format natural stone floor tile the bonding material must evenly and thoroughly cover the area beneath the tile.
Hang a batten board to prevent tile slippage.
A batten board is nothing more than a 1 x4 or 1 x2 board screwed directly into the wall horizontally.
Even with some patterned tile such as marble you can still accomplish this feeling if the pattern runs horizontally.
Large format tiles greater than 12 inches placed horizontally on walls tend to make a space feel bigger because there are fewer lines to break up the surface visually.
Place the tile in place and give it a slight twist as you place it in it s final location.
Hold it tight for about 20 to 30 seconds.
1 get a bunch of different sized spacers and use them according to what fits possibly using a knife to cut them in half.
In theory the easiest solution is to build from the bottom supporting the lowest tile course right on the floor and letting the upper courses rest on those.
Good technique is to pull a level line one tile up from the bottom of the wall.
Tips for tiling a wall.
Large heavy wall tiles are a challenge to hang because the weight of tiles makes them prone to slip off the wall.
The thinset should not let the tiles moves.
Use of a large trowel 1 2 x 1 2 inch square notch or 3 4 x 3 4 inch rounded notch will help ensure that the mortar adequately covers the substrate.
Two things have occurred to me.
Place a field tile at your low point with a thin spacer underneath tile wedges or round toothpicks.
If the bottom is not level then pick the low spot and measure one tile up from there and level over so you may need.