Remove the throw pillows and seat cushions from the couch. Unzip the seat cushion cover and open it. Place the seat cushions in the cover in the order you want them on the couch. Zip up the cover and set it to the side.
Drape the main body of the slipcover over the sofa. Start by draping the arms, then pull the slipcover over the back of the couch. Adjust the slipcover so the seams are in the correct places over the arms and over the back of the couch.
Push the excess fabric next to the arms down into the seat with the spatula. Smooth the fabric over the backrest and tuck any extra fabric into the crease next to the arms.
Put the covered cushions back on the couch. If your slipcover comes with ties at the bottom, tie them tightly.