Determine the measurements of the window-seat area, and select cabinets of an appropriate size from a kitchen-supply store or home-improvement center. You'll also need to buy molding that matches the cabinets.
Remove shoe molding from around the wall in the sitting area.
Build a wooden frame pedestal from 2" by 4" lumber (Image 1). (For added height, use 2" by 6" lumber for the frame.) The frame should be the same width as the window-seat area but 2" or 3" less than the depth of the window seats to create a recess underneath the front section. Include a center support in the frame for added stability.
Lay the frame flat inside the seat area, and secure it to the wall with screws (Image 2).
Remove the doors from the cabinets for easier handling. Clamp the cabinets together (Image 1) to ensure that they're flush on top and along the front.
Using a bit narrower than the screws, drill pilot holes in the side frames of each cabinet, and secure them with wood screws.
Center the cabinets between the two side walls (Image 2).
Drill pilot holes through the bottom of the cabinets and into the base pedestal. Secure the cabinet to the base with screws. Remember that the base is recessed. Mark a line indicating the location of the base to help you screw directly into the wood.
Measure the space between the wall and the cabinets (Image 1). Measure at the top and just above the base molding on the wall.
Transfer the measurements onto a filler piece (a piece of trim provided with the cabinet), and cut the piece to size. You may need to cut a notch in the filler piece to accommodate the base molding.
Set the filler pieces in place (Image 2), and fasten them to the sides of the cabinet with screws. Keep the top and bottom of the filler pieces flush with the cabinet.
Measure the area for the toe kick. Cut a piece of trim to size, and secure it with small finish nails. Drive each nail until approximately 1/4" of the head is above the surface. Use a nail set to drive the nails just below the surface.
Measure the length along the face of the cabinet. Transfer the measurements to a piece of shelving material, and cut it to size. Use shelving that matches the cabinets as closely as possible. Make sure the depth of the shelving is sufficient to cover or slightly overlap the front of the cabinets.
Cut a piece of cabinet trim to the same length as the shelving material.
Using the cabinets as a work surface, drill pilot holes through the edge trim and the edge of the shelving material. Secure the shelving and trim with small finish nails. Use a nail set as before, and use clamps to hold the tops of the boards flush.
Remove the boards from the top of the cabinets, and run a bead of panel adhesive along the cabinet edges. Press the seat into place.
Replace the cabinet doors. Fill any nail holes with caulk or matching fill material. Caulk any voids along the wall, and use trim pieces to cover any gaps between the seat and the wall.