Just get some chair rail molding or some type of 1 trim pieces.
Tiling uneven walls shower.
With the uneven ceiling level you would normally level or laser a horizontal line that runs around the shower enclosure at the top of the tile if leaving a top border tile free see below then measure your row lines down from there to the bottom and start with any bottom trip and cuts and actually place the tile from bottom up to the top.
When you ended the tiles at the window why weren t they all cut evenly.
This will provide a straight edge for the tile to follow and help support it while it dries.
Whatever you put there you are going to see the uneven tiles.
Draw a leveled line where the bottom of the second row will fall.
You could have wrapped around the edge w the tiles for a neater look.
Things you will need measuring tape tile wet saw leveling spacers and wedges trowel thinset mortar rubber mallet grout grout float grout sponge.
This is a dusty job so use plastic to cover your doorways wear a respirator that s rated for dust cover the floor with a drop cloth and make sure there s plenty of ventilation.
If the shower base is uneven tiles will have to be cut to fit the last row.
If any part of that should get wet you could have some issues.
I ended up going w a 1 thick basalt chair rail.
Attach a straight board to the backer board along the line.
You should have used the proper substrate for your shower walls.
You still may be able to do it by removing the last tiles and redoing it.
Begin by removing the old tile or the fiberglass shower wall.
Find a matching thick chair rail edging 12 piece and you ll be fine.