Measure along the walls and find an appropriate length for the bench seating. Cut two 2x3s for the frame to this length using a chop saw.
A cleat is a narrow piece of wood affixed to the floor and/or wall for structural strength purposes. Use a table saw to cut a 2x3 in half lengthwise, creating two cleats. Using a chop saw, cut these cleats to the same length as the attached 2x3s. Take the longer of the two cleats, and chop an additional 16 3/4" off. Finally, cut two 16" cleats and set them aside for later.
Measure out 16 inches from the wall on the floor (Image 1). Place your cleats on the floor at the 16" mark. Butt the single-cut cleat up with the wall. Line the edge of the cleat 16 3/4" off the far end of its corresponding 2x3. The two cleats should both be on the floor, 16 inches from the wall, with a 3/4" gap between the wall cleat and the cut cleat. Pre-drill and screw the cleats into place (Image 2).
Rip a sheet of 3/4-inch plywood into two pieces that are 18 inches high and as long as each corresponding cleat. These will act as the structural faces of your bench seating. Cut two pieces of plywood to 16" x 18" and set them aside for later. Slide the appropriate plywood face through the 3/4" gap in the cleats (Image 1). Notch out the corner of this piece so it will fit snugly in place.
Hold up the other piece of plywood in front of the other cleat. Square up the two pieces of plywood and drill through the "slide through" face into the edge of the other piece (Image 2). Span a level across the top of the plywood fronts and the 2x3s on the wall, and shim, if necessary (Image 3). Once the faces are level, use a screw gun to fasten them to the cleat (Image 4).
Place a 16" cleat, previously set aside, inside the open edge of the bench seat, and screw it into the floor. Hold the 16" x 18" piece up against this cleat and the edge of the front face of the bench seat. Make sure it's square and level, and screw it into the cleat and the plywood face. Repeat process for the other end of the bench seat.
Cut a 3/4" x 1-1/2" furring strip in half lengthwise to create two 3/4" x 3/4" pieces. Measure out from the corner of the bench seat to the ends. Cut the 3/4" x 3/4" to fit these lengths. Attach them to the ledger and wall with construction adhesive and a nail gun.
Measure and mark the center point on the furring strip (Image 1). Measure and mark the center point on the piano hinges. Align the center marks of each piano hinge and furring strip and mark out the screw holes with a scratch awl (Image 2).
Measure the horizontal depth and length of the bench. To create the lid, cut a 3/4" sheet of plywood to size, plus one inch for overhang. Remember, if you are making a corner bench, one of the lids will extend the full length from the edge of the bench to the wall. The second lid will only extend from the end of the bench to the other lid. Measure to find the center of the lid. Align the center of the lid and piano hinge, and mark the screw holes with a scratch awl. Place your seat/lids into place.
Measure to find the center of the newly upholstered lid. Align the center marks of the lid and the piano hinge. Screw the hinge into the lid. Hold the lid into place, align the center marks of the hinge and furring strip, and screw the hinge into place using the marked holes. Apply molding to the bench, if desired.