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Build a 24" high pony wall (tall) 18" away from the windows. Anchor it to the floor with tapcons.
Build a 4" pony wall (short) on top of the existing window ledge, both being at the same height.
Install 2x4 horizontal joists from the outside pony wall (tall) to the inside pony wall (short). Put 3/4" plywood on the top side as the seat and 3/4" plywood on the vertical face of the tall pony wall to form the bottom bench part of the seat.
Cut 1x6 pine to lengths of 18" and taper them from 5-1/2" to1/2". These will be the vertical studs of the back rest (Images 1 and 2).
Cut the top and bottom plates made of 1x6 (bottom) and 1x4 (top), and lay them out at 16" on center (Image 1).
Nail the plates to the studs to form the back rest and then install 3/4" plywood on the face (the part your back rests on) (Image 2).
Before installing it on top of the bottom part of the bench, install 1/4" plywood on the face that looks at the street (Image 3).