A sheet of thick Styrofoam or foam insulation board serves as the main structure for a homemade window valance, requiring no sewing whatsoever. Since this valance weighs little, it also won't require wall anchors to keep it in place. Choose a decorative fabric that complements the decor of the room; an upholstery fabric adds a little weight and sophistication, making the valance seem like it came straight from a designer's studio, custom made for you.
Measure the window's width, including frame or trim.
Set the scrap cardboard on the work surface to protect it from the utility knife. Set the Styrofoam sheet atop the cardboard. Add 1.5 inches to the measurement from Step 1 and mark the sheet along one edge using the tip of a utility knife, or by pressing down with a pencil. Mark the same point along the opposite edge of the foam.
Place a straightedge between the marked points on the foam sheet. Score the sheet using a utility knife, slicing over the line several times until the cut goes through the foam. Separate the pieces. The large piece is the front of the valance; the other can be saved for another project.
Plug in the hot glue gun and insert a glue stick. Once the glue is hot, apply a line of glue along the long and thin side of one of the matching narrow foam side pieces. Press the foam on top of the valance foam along the edge, aligning the edges of each piece perfectly. The construction should look like an L.
Apply a line of hot glue along the long and thin side of the other foam side piece. Press it in place at the opposite end of the front valance board, mirroring what you did with the first piece. The construction should look like a straight C or a staple.
Push eight to 10 straight pins through each of the glued side pieces and into the main board to help secure the pieces, much like nails secure boards to one another.
Set the decorative fabric face down on the work surface. Spread the batting out atop the fabric, smoothing it with your hands.
Place the foam guts of the valance face down atop the batting. Fold the fabric and batting together up around the back of the valance and around the support arms as if wrapping a gift. Press pins through the fabric and into the foam at an angle every inch or two to hold the fabric and batting onto the foam; the angle helps ensure the pins won't poke through the other side. Tuck and wrap around corners of the foam supports as if wrapping an oddly shaped gift, securing the material with pins pushed in at an angle. Cut away excess fabric using scissors.
Hold the valance up over the window frame, or enlist a friend to hold it, while you push pins through the side foam supports into the window frame to hold the valance in place on both sides.