Measure the length and width of the window opening, inside the sliding tracks. Multiply this number by two to determine the total perimeter of the window. Multiply the perimeter by 0.05 to determine the total amount of window felt needed.
Measure the width and height of the current weatherstripping. If no weatherstripping is installed, measure the size of the gap around the window in the frame to determine the necessary size of the felt strips before purchase, as weatherstripping felt comes in a variety of widths and thicknesses.
Remove the old weatherstripping, if applicable. Wash the interior of the window frame with soapy water. Rinse with clear water and wipe dry completely. The felt adhesive only adheres to clean, dry surfaces.
Peel the paper backing from the felt's adhesive, peeling up only 6 to 8 inches of the paper. Set the adhesive against the window frame and press the felt into place.
Continue peeling up the backing and pressing the felt into place, working in small increments so the adhesive doesn't become dirty and to prevent the felt from sticking to itself. Work around the entire window frame until the felt completely surrounds the window opening.
Trim off the excess length of felt. Press the end in place so it sits snugly against the other end of the felt strip.