The back legs of the bench will require some custom cutting. If you have a band saw, cut the two legs at the same time by screwing them together. Otherwise, cut the legs individually with a saber saw. For strong joints, use wood glue and thin wooden wafers known as biscuits. You'll need a biscuit joiner to cut the slots for inserting biscuits.
Wooden dowels are an acceptable alternative to biscuits.
Lay the seat and side supports in place, and mark the locations for cutting the biscuit joints. Clamp the pieces in place until the glue has dried. Be sure to wipe up any excess glue before it dries. Dry-fit the slats for the back of the bench, and mark the locations for joint cuts. Make sure that the pieces are square, and clamp them together.