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.
Tiling uneven walls shower.
Begin by removing the old tile or the fiberglass shower wall.
Mine is the same way because i used 3 4 thick tile on my wall.
I ended up going w a 1 thick basalt chair rail.
Whatever you put there you are going to see the uneven tiles.
Just get some chair rail molding or some type of 1 trim pieces.
When your walls are uneven and you use very thick tile which yours is it s going to happen.
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.
When you ended the tiles at the window why weren t they all cut evenly.
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.
You still may be able to do it by removing the last tiles and redoing it.
Things you will need measuring tape tile wet saw leveling spacers and wedges trowel thinset mortar rubber mallet grout grout float grout sponge.
This will provide a straight edge for the tile to follow and help support it while it dries.
Find a matching thick chair rail edging 12 piece and you ll be fine.
Also tile is heavy so put down blankets to protect the floors and tub from being damaged by falling pieces.