Take the measurements of your couch and purchase the smallest one-piece slipcover that will still fit it. The smaller size will keep the slipcover from looking saggy. Choose a slipcover with ties to help it fit snugly and elastic that holds the fabric under the cushions to make it look cleaner.
Drape the slipcover over your couch with the cushions on. Line up the sides, back and front with the couch and then tuck the slipcover in between the back of the couch and the cushions. Tuck the sides in between the cushions and couch arms.
Straighten the slipcover fabric and tie the fasteners. Replace any accent pillows.
Purchase a custom-fit slipcover to fit your sofa. Contact your manufacturer to see if they make slipcovers. Many furniture stores such as IKEA and Pottery Barn make slipcovers that fit perfectly over their sofas, with separate covers for each cushion. Check the model of your sofa and purchase a slipcover to match.
If the manufacturer does not make slipcovers for your sofa, contact a company that makes custom slipcovers. The company will instruct you how to take measurements and choose fabric. They will need to know that the cushions are T-Shaped. Be sure they know the height, width and length of each cushion as well as the height of the arms, back and front of the couch. You may also look for a tailor in your area who makes couch slipcovers, who will be able to take measurements.
Once you have your slipcover, remove the cushions and the existing slipcover, if necessary. Place the slipcover on your sofa. Put your cushions into their covers. If the existing fabric shows through, you will need to remove it and then put the cushions in your new slipcovers. Zip the cushion covers and place the cushions on the couch.