This is like writing out how to tie a shoe...I'm not sure I'm up to it, but here goes.
The sail cover for the furled genoa is rectangular in shape, 12 inches by 30 feet, made of sunbrella pieces sewn together. Along the long edges are the two sides of a zipper -- actually two zippers, one that goes up the sides for about 5 feet from the bottom, a small gap, then another zipper that runs to the top of the cover. Also at the top of the cover is a grommet.
Tie the end of the old jib halyard through the grommet, wrap the top of the cover around the sail, start the zipper -- and hoist a bit of the cover and zip, hoist and zip until the cover's to the top. Bring the jib sheets out through the gap, coil them and hang on the sides of the bow pulpit, engage the bottom zipper and run it up to the gap. Cleat the furling line, tie a hank of line around the cover and sail for extra security. Presto, a covered sail.
It's easy, clean and shippy.
--Dave