Look under the seat and unhook all the S hooks holding the cord in place. Since you won't be needing the old cords, they can be cut to untie any knots.
Remove the headrest from your car's seat by pulling it up and out of the seat. Push the back of the seat forward, toward the front of the car.
Pull the old cover off the seat and discard.
Place the new seat cover over the top of the back of the seat and pull it down. Stretch the cover out on all corners without leaving wrinkles in the material.
Fasten the elastic under the seats using the S hooks that came with the packaging. Attach the hooks to the framework under the seat cushion. The framework is visible underneath the seat.
Tie the new cord to the framework so that it pulls the seat cover snugly against the seat.
Use a razor blade or knife to cut a small slit in the seat cover directly over the holes for the headrest. The holes will stretch, so make sure they are small. Insert the headrest and adjust it accordingly.