Begin Framing the Wall
To begin framing the wall (Image 1), first add a bottom plate, minus 1/2" for the drywall.
Measure for the end piece — which will go from the end of the ledge in this case, and back to the exterior wall — minus 1/2" for the drywall.
Install the first stud against the wall next to the door frame. Use a level to ensure it's plumb.
Install the angled top plates on the ceiling (Image 2). Insert 4" toggle bolts into the predrilled 2 x 4s and insert into holes into the ceiling. Tighten the bolts.
Cut the studs for the walls. The tops of the first two are cut at an angle and at different lengths that the others. The rest of the studs should be about the same length.
Install the cleat (essentially a 2 x 6 brace) on the exterior wall. Use jack studs underneath to strengthen and support the cleat.
To secure the front braces, add jack studs and nail in place (Image 3). Add 2 x 6 joists that span the braces (Image 4), then cover the joists with 3/4" plywood shelf (Image 5) to support the tank.