Create a rectangular frame from the bench-side aprons, 60-inch, 1-by-3 lumber, and the bench-end aprons, 8-inch, 1-by-3 lumber. Position the ends of the end aprons so they're inside the ends of the side aprons.
Attach the aprons frame together with two 2-inch deck screws through the side of the side aprons into the ends of the end aprons.
Insert five support planks of 8-inch, 1-by-2 lumber evenly across the frame, with the support planks pushed down to be even with the bottom of the aprons.
Fix the supports in place with two 2-inch deck screws through the side aprons into the ends of the supports.
Lay the bench top--a plank of 72-inch, 2-by-12--face down on a work surface. Center the frame on the bench top with the supports touching the underside of the bench top.
Secure the bench top with four 2-inch deck screws driven through each support plank into the underside of the bench top. Leave the bench and frame upside down for now.
Insert a bench leg, a 17-inch plank of 4-by-4 lumber, into each corner of the frame. Fix each leg to the frame with four 3-inch lag bolts through each side of the frame into the legs; eight bolts altogether into each leg. Turn the bench over and finish with a wood finish of your choice.