In a pinch you can throw a large flat sheet over a couch to cover stains or damage. The sheet should hold up for a few hours. When possible, smooth the sheet out over the seat back and tuck it between the seat cushions so that it won't move a lot when people get up and down off the couch. Remove the sheet as soon as possible, and repair or clean the couch cover.
Sheets can be used to make slipcovers that are fitted. The sheet fabric will still be thin and more vulnerable to stains and damage than more durable fabrics, but the customizing and fit of the cover to the sofa will greatly reduce the stresses put on the fabric. Cover the largest sections of the sofa first with the sheet facing the sofa, and pin 1/2-inch seams. Fit the seat base to the floor second, and fit the arms last before sewing your seams. Make individual covers for each cushion. You may need two large sheets to cover an average sofa and two more for the cushions.
Quilt two sheets together with a thin layer of batting for a more substantial cover. By doubling the fabric and adding the batting, the fabric will be much stronger and more comfortable. You can make a slipcover that will cover the sofa and cushions. The fabric will be more durable and the thickness of the quilted material will hide the seams of the cushions for a sleeker look.
Purchase king size flat sheets when they are discounted. Seam two sheets together in the middle, and repeat this for a second set. You can use one color for each set to create a double option cover. Quilt the two sets together. Quilting just means sewing a pattern over the materials to join them together. Center your large sheets on the sofa, and tuck the fabric deep at the seat back. Use rolled magazines to keep the fabric in place. Smooth across the seat, and fold and tuck the arms. Bring the extra fabric to the back, and pin with large safety pins to create a semi-fitted sofa cover.