When installed correctly, a vinyl J-channel provides protection from water and creates an attractive trim for windows. Installing the J-channel requires tin snips, a tape measure, a utility knife, a hammer and roofing nails.
Cut and install the bottom J-channel
Measure the width of the window. Add 2 inches to that number and cut a length of J-channel to this measurement. Score a 1-inch notch along the top side of the J channel on each of the ends. Use the tin snips to cut into the corner of the scored notch on a diagonal. Break off the resulting pieces. Place the J molding along the bottom edge of the window. Loosely nail every 8 inches in the nailing slots.
Cut and install the side J-channels
Measure and cut two strips of J-channel 2 inches longer than the height of the window. Notch out 1 inch of the inside top and bottom edges. Overlap the notches with the bottom section and nail into place.
Cut and install the top J-channel
Cut the top J-channel 2 inches longer than the window's width. Cut a 1-inch notch with the tin snips, leaving a tab on each end. Overlap the top and sides and nail the top trim in place. Fold each tab over each side piece.