Laying tiles in the toilet - How / Where to start?

Tiles are too dark. Size is okay, but there are better ones!

Regarding your questions:

You cut tiles with an angle grinder. Diamond blade on it and off you go. You break the rest apart easily with a tile cutter. Baseboards cover unsightly edges.

Take a 2m straightedge and see how big the unevenness is. Up to about 3mm you can still even it out with tile adhesive, if it's more than that then use leveling compound once!

You should think carefully about how you lay them beforehand. Measuring and graph paper is enough. Joint width is best 3mm up to max 5mm.

I have tiled a lot in my life, if you have questions just get in touch again.

For beginners, I always recommend a laying system to keep it level. Otherwise, you will certainly get uneven joints and height differences. Later you can work without it, using a rubber mallet and spirit level.