Measure the height and length of the couch. Measure the height and length of the cushions and add those measurements to your overall height and length measurements. Add another 4 inches all around to allow for tucking the slipcover into the couch's seams.
Purchase the appropriately sized sheet for your couch, based on the measurements you made. Drape the sheet evenly over the front of the couch so that 1 inch of the sheet is lying on the floor and an extra 1 inch is at the top of the couch.
Tuck the sheet around the couch cushions and the corners of the arm rests. Tuck the excess sheet at the bottom of the couch under the couch and the excess sheet at the top of the couch behind the top of the couch.