How do I tile around my tub and shower when the corners are not square

I just had this same problem myself. I hope by drywall you're referring to green board, if not cement board. If you're still able, try shimming out the spots that need squaring up. If it's too late and you've started tiling, you'll either have to hope the lack of squareness is not noticeable or start over.

Since the drywall is up all you can do is make the corners disappear visually. Set up row of tiles leaning on tub from center to corners on main wall. Move the tiles back and forth along the wall. Make sure both corners have the same gap. Those will be your cut tiles. Start gluing the tiles to the wall from your center towards the corners. Do not cut tiles for the corners until whole wall is covered. Then just measure corner tiles, cut tiles, and glue. On adjacent walls start gluing full tiles from a starting point away from corners towards the corners. Corner tiles will need to be cut to fit. Do not cut tiles until whole walls are covered. Again, measure corner tiles, cut, and glue. A bead of bathroom caulk in the corners and the not square will blend in visually.

In the future make sure corners are plumb and square using wood shims on studs behind the drywall. No corners are 100% perfect. Houses move and settle. Wood expands and contracts.

https://www.google.com/search?q=tiling+corners+that+are+not+squar&rlz=1C1CHBF_enUS750US750&oq=tiling+corners+that+are+not+squar&aqs=chrome..69i57j33l2.11851j1j7&sourceid=chrome&ie=UTF-8

Hi, sadly no walls or floors are ever truly square, hope these help

https://www.youtube.com/watch?v=6XDPOioPDWs

These are finishing strips for use with tile in unsquare situations

https://www.schluter.com/schluter-us/en_US/

You will need to make a template of the area tile by tile and use that to cut each tile to fit . You can get tile cutters (buy or hire). Or call in a tiler to dpo the job for you!

https://www.amazon.com/rechargeable-light-bulbs/s?k=rechargeable+light+bulbs