Arrange the sheet(s) on the sofa so it is fully covered to the floor. You should have a lot of excess fabric.
Tuck the fabric into the middle of the cushion pads and back pads if there are any. Otherwise move to the next step.
Smooth the fabric on the seat flat and tuck between the seat and back cushions. Smooth the sheet flat across the seat and tuck the excess fabric between the seat and sofa arms.
The sheets should be resting on the floor on either side of the sofa arms.
Be sure the fabric hanging over the back of the sofa completely covers the back and pools slightly on the floor.
Pull up the fabric that overhangs the seat cushions in the front so the bottom edge pools slightly on the floor and tuck the excess fabric under the front of the seat.
Drape the excess fabric of the sheets around the sofa arms in soft folds.
Cut a length of blanket binding and wrap around the bottom of the sofa to hold the slipcover in place. Tie in a bow to finish.