Right, take two ?
Typed under the assumption you have a laser level. If you haven't, then buy one lol. Anyhoo...
Id go landscape with that size tile but it's not my ( or your ) bathroom ( it's swmbos ) so see how that works out. .
Your bath is the focal point, so you need to get the cuts looking good on that first. Ping a line around the room from the horizontal deck of the bath, and see where the bottom tile will end up at the shower corner, holding a tile on the line and counting down. Your right, you'll need to extend down past the floor / wall junction to reach the lowest point of the wet floor so see how that ends up looking.
Once you have established a happy medium between the bath top edge, and the shower bottom edge you'll have a datum line for your first grout line from the floor. Use a tile to mark each grout line up the wall and then grab the laser and sit it on each line from floor to ceiling. Check the lines for conflicts with things you want to tile to / around, like the pocket shelves and raise lower it according to how much play your first line-out will allow.
I always set the pocket shelves out, where possible, to be divisible between one or two tile courses, so you have factory edges to set the chrome trim against. See pic. Those trims are against the uncut tile edges and it makes life, and tiling, much easier and neater of you can work this all in together.
Iirc, you've already made the boxes for the shelf to reside, so all the more important to carry out my first rule, which is to set out and ping lines around to get the best datum / start point, according to which one will NOT give you a tiny rip at the top of the bath or around the pocket shelf.
After that, check the window for cuts and then repeat with the vertical, again checking that you'll not end up with any near misses or small slivers of tile.
Make any sense?