Don't let yesterday's rainstorm dampen your plans for a picnic. You can waterproof a cotton or wool blanket by adding a protective backing of water resistant fabric such as ripstop or oilcloth, and safely lounge on damp grass
Cut fabric to the exact dimensions of the blanket,adding 1/2 inch on all sides for hem. Fold edges of backing under 1.2 inch, and place 2 inch wide twill tape on top;machine stitch both inside and outside edges of tape around entire perimeter,mitering tape at corners.
With a hammer or mallet, fix snaps along edges of the backing and the blanket. Center a snap in the twill tape at each corner,then attach snaps at intervals of 10-12 inches around the whole border. Attach the male half of the snaps to the backing first, then mark the placement of the female half of snaps carefully on blanket for a good fit, and attach snaps. The backing may be detached when it is not needed.