Layer the two sheets of fabric evenly, one on top of the other.
Sew all but one of the 80-inch edges of the fabric sheets together. Make sure to only adhere the fabric's edges.
Line up the zipper tape with the open edge of the top layer of the fabric. Make sure the zipper side of the tap is facing downward, with the vinyl cloth tape overlapping the edge of the fabric by 1/2 inch.
Make a tight stitch down the entire 80-inch length of zipper tape, adhering it to only the top layer of the fabric.
Flip the fabric over to expose the bottom layer of fabric, making it the top layer.
Unzip the zipper tape 3/4 of the way down the side. This separates the zipper so that you can adhere the other side to the secondary layer of fabric with ease.
Line up the edge of the loose layer of fabric with the fabric edge of the opened side of the zipper tape. The fabric edge of a length of zipper tape is what you adhere to fabric to hold the zipper in place.
Sew an even seam down the length of the sheet and zipper tape.
Turn the futon cover inside out to conceal all of the seams, and to make the zipper side of the zipper tape show.
Slide the cover over a full or queen-sized futon pad or mattress.