Measure the width and depth of your window nook to determine the dimensions of your desk surface. Record these measurements on a scrap of paper.
Determine the height at which you want your desk to sit. Pull a desk chair up to the window nook, sit down and hold your arms out in front of you at the height you find most comfortable for work. Drop the end of a measuring tape down to the floor to measure the height.
Draw a horizontal line on the walls on either side of the window nook at the height you just determined. This will provide you with a guideline for mounting your wall brackets.
Position four large, L-shaped metal wall brackets -- two on each wall -- so the corners are aligned with the line on the wall. The two brackets on each wall should be placed 7 inches from the window and from the edge of the wall. Mark the intended position of the brackets in pencil as well as the position of the screw holes in each bracket.
Pre-drill holes in the wall at each of the wall bracket screw hole markings. Insert the tip of a plastic wall mount from a wall mount screw set into each hole and tap it gently with a hammer to push it into the wall so the head is flush with the wall.
Align the wall brackets in place, one by one, and insert a screw through each screw hole and into the corresponding wall mount. When finished, your four wall brackets should form a horizontal frame on which to mount your desk top.
Cut a piece of 1-inch plywood to the width and depth measurements you took earlier. Sand down the edges with fine-grain sandpaper and lay the plywood flat on top of the wall brackets to form the desk top.
Crouch under the desk top and mark the position of the screw holes in each wall bracket on the underside of the plywood in pencil. Remove the plywood and pre-drill a hole at each marking.
Replace the plywood on top of the wall brackets and insert a wood screw into each of the pre-drilled holes to secure the desk top in place.
Pull your favorite desk chair up to the desk and arrange your office supplies as you desire. Decorate your window nook by hanging small pictures or framed quotations on the wall to define the space.