Tile edge trim installation is always my first shower tile installation step.
I suggest that you NEVER attempt to wedge the trim under the vertical leading edge of your wall tiles while you are installing them. There are so many things that can go sideways with this strategy if you don’t have a lot of experience setting tile.
Instead, you should determine the distance (horizontally from your tub or shower pan edge) where you would like your wall tiles to extend. This is where you will mount your tile edging.
I would suggest that you try to place your edge trims in a position that will allow you to lay one complete horizontal row of tiles with no tile cuts, if possible.
Now draw a vertical pencil line on both sides of your shower enclosure to trace where you’ve chosen to mount your trims.
Then simply glue them in place using a tiny amount of polyurethane adhesive every 6″ or so along the trim.
Further secure the trim to the wall with 6-8 wafer head screws to allow the trims to follow the undulations in the wall (as long as they are not too severe). The screws are removed the next day when the adhesive dries.