Once the paint fumes cleared, it was time to frame the walls. After a bit of trial and error, we settled on running 2x4s horizontally along the base of the wall and underneath the windows. Then we cut vertical support beams, which we notched to fit around the steal beams that the seats were attached to. We used 2" self-tapping metal screw to attach the studs to the walls. Incidentally, "self-tapping" metal screws work a whole lot better if you drill a pilot hole.
We also started cutting the framing the shower. We're using a Dreamline 32" x 32" shower base.