Extend a Windowsill: Select plywood the same thickness as sill; cut it as wide as sill and 12 inches deep. Attach bottom of shelf to bottom of sill with 3 hinges: 1 at the center, the others near the sides. Cut 2 right triangles from wood thin enough to lie flat beneath folded shelf; trim tips. Brackets should extend 3/4 of the way across shelf and 3/4 down apron. Hinge brackets to apron. Paint before installing.