You need to block with wood, every area you will be mounting hardware. That includes grab bar, hooks, door hinges, baskets. I used an old scrap piece of lumber for my towel bar.

Blocking around the toilet for grab bars in the back and to the side. Also a block for the toilet paper fixture (large area at the very right of the photo):

Didn't end up using the option for behind the toilet, but blocking is there if needed in the future:

Area at my shower: