If your shower pan is not level then the level line you created to mark the top of the first row will mean that some of your tiles need to be cut.
How to cut tile on shower wall.
Cut through the tile using an angle grinder.
Set the depth of the blade to 1 4 inch.
Start at one end of the cut line and run the blade along the tile surface to the other end.
You could use a score and snap tile cutter or a wet saw with a diamond blade.
Push your tile firmly into place.
Hold the saw up to your marked lines.
Getting stuck and not having an answer well that s my biggest fear.
Once only the bottom row is remaining remove the support board.
Cut away large sections or large cutouts with a handheld circular saw equipped with a tile cutting stone blade.
Or you can use a 4 1 2 angle grinder with a tile blade.
Cut as needed with tile snips or a wet saw to allow tile pieces a better fit around shower faucets and shower heads.
Spray water onto the tile when needed to keep the surface wet.
Use spacers between the tile to make sure the gaps are consistent.
You can buy a 3 or 4 carbide hole saw but they are super expensive.
For exposed edges set edge trim or bullnose tile pieces in place.
Periodically check the tile line with a level to be sure they are straight.
Use bullnose for exposed edges or you can use trim pieces.
This tends to happen when homeowners need to cut a large hole in tile.