I first saw these benches at Brimfield a few years ago. I couldn't wait to make my own. First you have to find a headboard and footboard. They seem to abound, especially the odd sizes.
This youth bed was perfect.
Cut the footboard in half. I used a jigsaw. Then I bought some knotty pine boards; 1x 2" for the ledger boards and 1x 4" for the apron and seat.
I started by adding ledger boards on the footboard/side pieces. I placed them just a board depth down from the the top of the horizontal boards. This allowed for the planks I added for the seat.
Next, measure from the bottom of the leg to the bottom of the ledger board. Use that measurement to position the ledger board on the headboard. Glue and screw the ledger boards in place.
I attached the sides by putting screws in from the back of the headboard. Predrill to avoid splitting. To stabilize and square the corners, I added 4" pieces of wood with 45˚ angles on either end with screws.
Tuck this in a shaded patio, at the end of a bed or just about any place you could use some extra seating.