Next, I'll build the framework for the seat. I'm using 1"x4" pine (It's what I had laying around), although this is what I'd use anyway. Since the front of the bench legs are pointing out slightly, I'll get my width measurement from the back where the legs are attached to the headboard.
I can't remember off hand what my measurement was but I'll cut two boards at this length. After stapling the first one one and into position, I'll set the other aside for right now and measure for my deep. Given these legs are a little curved on the ends I'll find a good place for my depth to end. If the legs were square or I was working with flat surfaces, placing this would be easier. For me, it's easy to imagine where this end and how it will look when done. For others, it may not be, so be afraid to hold up some pieces just to see how things will play.
Once I have decided on my depth, I'll take a measurement remembering to subtract 3/4" (width of my front board). After cutting those, I'll attach those making sure they are square to my back board and level to my legs. Next is installing the front board, thus creating a box. I'm still not done, I plan on putting a thin piece of plywood over this but I need something to support that or things may sag over time. So, I'll create some slats that I'll mount at 1/3 and 2/3 my width.
When completed you should end up 3 boxes, everything level and true.