Place newspaper or some other covering you don't mind dirtying underneath the sofa if you are worried about getting debris on your floor.
Remove the sofa cushions and any fabric covers that are removable from the sofa.
Lay the sofa on its back so that the portion you sit on is vertical.
Put on your safety goggles and plug in your electric saw. A saber saw will work best, but you can also use a circular or heavy-duty jigsaw.
Use your saw to carefully saw down the center of the couch.
Work your way through the seat and then through the back of the sofa. Once you cut through the back the sofa will be cut in half.