I just did this over the past few days. My house was built in 1840 so it's out of square in more than one plane. It wasn't too hard at all, just time consuming. Here's what I did:

Measure each wall height, divide by two and make a mark, then use a level and draw a line floor to ceiling (or to wherever your tiles will stop if it's not all the way to the ceiling). Do the same horizontally.

Now you've got a grid of sorts to work from, with four quadrants on each wall. Lay out your tile dry so you can see how they fit, making sure you won't end up with little slivers on the ends - you don't want to have those bits especially in a corner. If you do, adjust your lines until you have a reasonably sized tile at each end of each row.

Now pick a quadrant to begin, start at the center of the wall and tile outward using your level line as a guide. When you reach the edge of each row, cut your tile to size. I start on the largest wall, and do the smaller walls last.

Some tidbits to bear in mind: leave a sliver of room at the ends of your rows, and don't butt the tile against anything - not the ceiling, floor, etc or they won't have expansion room and you'll have tiles popping out down the road. Use spacers to ensure things line up properly. 1/8 spacing works for most smaller tile. White grout gets gross, try to avoid it. Most grout can discolor if you do it too soon, wait like 24 hrs to ensure your thinset is dry. Caulk all transitional spots like corners, along the shower base, etc instead of using grout there - they make caulk that'll match your grout.

This should get you nice level rows, and once you caulk the corners things should be whacky looking. If your top row ends up with an awkward gap at the ceiling, crown molding works great to cover that.

Lots of pros will tell you not to use mastic on a shower wall. There's a ton of excellent resources online on wall prep, how to spread thinset, pretty much everything. YouTube is great for this.