Lots of people make templates for us using thin MDF or hardboard.
1. Take the bottom 2 or 3 courses of tiles off the walls. This may destroy the wall boards if they are plasterboard so you'll need to be careful and a tool like the Fein Multimaster will be invaluable.
2. Rake out the old and damaged wall grout and make space in the corners for a silicone bead as there's no grout in here.
3. Remove the existing floor up to a convenient grout line, fit new, level ply where the old floor was removed and then template for the new tray to fit under the wall boards.
4. Fit the new tray and make good the wall boards using Wedi boards with a 200mm strip of Wedi board glued behind the plaster boards to provide an overlap which will strengthen the joint between old and new. The Wedi boards will need to be notched in the back at the bottom to fit over our tray's drip stop lip.
5. Re-tile the bottom couple of rows leaving a 2-3mm silicone gap between the new tiles and the tray. This allows for a wide enough silicone bead to allow flex between the floor and walls; if you go too thin the silicone doesn't have enough flexibility.
6. Re-grout then silicone seal the wall corners and wall to tray junction.
Please also see: