I built 2 of these benches using different sets of reclaimed legs, but the process of building them was the same. I used my Kreg jig to join all the pieces. I basically built a coffee table base, using 1x4's for the aprons. I added a 1x4 stretcher bar between the front and back to give added support and diagonal blocks in the corners to keep it square. I added some strips of shoe molding the the bottom of the aprons using glue and 3/4 in finish nails. I then flipped the base onto a piece of 1/2 in OSB and traced the shape for the seat. Using a circular saw, I cut out the seat.
I pre-drilled holes for my tufting and then attached my foam. Using batting and upholstry fabric and staples, I covered the foam. For the tufting, I used a 7i n needle, covered buttons, and upholstery thread. Once I feed my needle through the pre-drilled holes, I pulled the thread very tightly, and secured the the back of the OSB, in a zig-zag pattern, using staples. I pocket holed my aprons and attached the seat.
I feature other projects like this on my blog http://pneumaticaddict.blogspot.com .