After all the doors are mounted, the bottom panel can be cut. If you are using base molding, cut the bottom panel 1" short to alleviate any floor variance issues. Mount another row of 2x4s 1" from floor. Run a bead of construction adhesive along both rows of 2x4s. Install the lower panel with 15-gauge finish nails 6" on center, 1" from the top and bottom. Reinstall any wall baseboard to this lower panel. Next, mix up a can of automotive body filler and fill any large holes from removed door hardware*. While body filler hardens, mount the ceiling crown. The lower edge of the crown will overlap the top of the doors enough to cover the mounting screws. Next, cover the butt joints between the doors, lower panel and walls with 3/4" flat screen molding. This forms a batten strip that complements the paneling but also conceals and gaps.
Body filler is meant to cover small imperfections. When using it to fill larger holes, underfill the hole. The filler will shrink (and possibly crack) as it dries. Instead of fighting this at the finish layer, let it dry, then go over it with another thinner layer that is slightly proud from the wall. Once this second layer dries, sand flat to the wall.