Lay a single plank of 15-inch 1-by-12 lumber flat on the work surface. Apply wood glue along one of the 12-inch edges and abut a second plank of 15-inch 1-by-12 lumber alongside the first, so they become glued together, forming a single plank, 15-by-24 inches. This is one end panel for the window seat. Set into F-clamps to hold while the glue dries.
Repeat with another two 15-inch 1-by-12 planks, so you have two end panels. Repeat with four 60-inch planks of 1-by-12, so you have two panels measuring 24-by-60 inches for the front and back of the window seat, and with two planks of 62-inch 1-by-8 lumber, so you have one 16-by-62-inch panel for the lid of the window seat.
Lay each panel flat on the work surface. Evenly space three planks of 22-inch 1-by-2 lumber across the joint of each of the front and back panels, and two pieces across the joint of the two end panels. The ends of these planks should be an inch in from the edges of the panels. Evenly space two planks of 12-inch 1-by-2 lumber across the joint of the lid panel, the ends of these planks should be 2 inches in from the edges of the lid.
Secure the 1-by-2 lumber supports in place with eight 1 1/2-inch wood screws driven through each of the supports; four screws on each side of the joint in the panel.
Miter the 24-inch edges of the front, back and two end panels to 45-degree angles on a miter saw. This will enable you to form a rectangular box from the panels, with the mitered edges mating to make the corners.
Apply wood glue to the mitered edges of the box panels and form them into a rectangle box and set into corner clamps to hold the shape as the glue dries. The supports should be on the inside of the box.
Apply wood glue to the edges of a 13-by-22-inch piece of 3/4-inch plywood and fit the plywood into the bottom of the box. It should fit snugly. Toe-screw (screw at an angle) the plywood base panel to the sides of the box from the inside of the box. Place one screw every 5 to 6 inches around the perimeter of the base.
Attach a decorative cutting bit to your router and cut around the edges of the window seat lid to add a ornamental element to the window seat.
Sand the window seat box and lid with a sander and, where the sander can’t reach, with sandpaper.
Finish the box, inside and out, and the lid of the window seat with the wood finish of your choice. Follow the application instructions from the wood finish manufacturer.
Center a piano hinge along the back edge of the window seat lid. Secure it in place with 1/2-inch wood screws through the pre-cut screw holes in the piano hinge. Place the lid on top of the window seat box, centered. There should be an inch over at the front and sides, but the back should be flush with the back of the box. Secure to the back edge of the box with 1/2-inch screws.