Put a vapor barrier on the studs, lap it over the tiling flange of the tub. Leave the bottom edge of the cbu about 1/4" above the tub and caulk that gap. Assuming you are going to tile this, use alkali resistant mesh tape and thinset. If there are areas that are just going to be painted, some people like to do a skim coat of drywall mud over the whole thing. You can use regular paper (stronger) or fiberglass tape on the corners prior to painting. Getting the screws set well enough to hide in areas that are going to be painted is a little tricky since the stuff is very dense, but possible if you use the special screws (they have ribs on the bottom of the head to ream out room to allow it to sink in).