Quite some time ago I asked about materials and waterproofing concerns for a shower window. Here: https://www.houzz.com/discussions/6220259/not-much-jamb-to-work-with-shower-window
I have finally completed the tile/sill/trim/grout.

I do not like asking too many questions on forums - but it is part of the process of figuring things out. Hopefully those who took the time to help will see these pics. This is a duplicate post from Terry Love's plumbing forum.

I put in a temporary frame that I could tile up to, and I had the 3/16 spacers between the frame and the tile. The spacers aren't seen because there was some fudging involved. The spacers were a guide.

The temp frame was removed when I set the top tile because it was not strong enough to hold the tile. A stick was cut to hold the tile.

Widow sill was set and the PVC trim was attached. PVC cuts great but is unforgiving when bumped around. Also, for the price, each piece should be pristine - they are not.

Sill and trim were taped off to be flush with the surface of the tile (1/8 around the side of the trim) and covered . No pic - sorry.

Then the grout was mashed in all around and the tape was pulled off during the first wash.