Step 1: Plan It Out
How deep you'd like your seat to be is the next question. I build this one to be 24" deep so we could use a rug runner instead of cushions. Runners are 21"-23" wide. How deep your seat is will also determine how wide it will be. At 24" deep, mine turned out to be 8' 5" long. Since sheets of plywood are 8' long it would have been easier to make the front shorter than 8' long and let the depth work itself out. In that way you could rip an 8' piece of plywood for the front whereas I used 2 pieces.
On www.askthebuilder.com I found a sketch of the framing for a window seat (pic 2). The height they recommend is 19" with a 21" deep seat. You can see from the drawing that a window seat is actually a short floating wall achored to 2x4 cleats on the wall. The front can be wood, sheetrock or other materials since it is only decorative. The one thing I did differently from the drawing was to place the 2x4s on edge as opposed to typical wall framing. This makes a sufficiently strong wall and also creates a bit more storage space inside the bench.