Attach brackets and a decorative pole rod to your window.
Drape a measuring tape from one end of the rod to the other, adjusting for the valance's swag in the center and tail length on the sides. If your measuring tape is not long enough, use a length of string. Measure the string. Add 24 inches for the rosette corners.
Cut a length of lace or sheer fabric, which looks the same on both sides, using the draping measurement.
Lay the fabric on a flat surface. Gather the fabric from one long edge to the other using your hands.
Insert one end of the gathered fabric behind the rod and against one bracket. Pull the tail down to 6 inches longer than the desired tail length. Drape the fabric length across the window on the front of the rod. Insert the remaining end behind the rod and against the remaining bracket. Pull the tail down to 6 inches longer than the desired tail.
Lift the fabric 6 inches on one end of the rod where it folds over the rod. Using your hand, gather the fabric below the fold, just above the rod. Place a rubber band over the loop, replacing your hand where it is gathered. Do not double-wrap the rubber band or the valance will not lay smoothly over the rod. Repeat on the other end of the rod with another rubber band.
Adjust the folds of the drape on the valance between the ends. Note: This may require adjusting the loops on the ends and the tail lengths.
Open the folds and gathers of one loop to form a pouf. Poke the fabric pouf's center into the center of the rubber band to create the rosette. Repeat on the opposite end with the remaining loop.