Remove the cushions from your sofa. Measure the width and length of the space beneath.
Acquire puzzle mats sufficient to cover the dimensions you measured. You can buy puzzle mats in different levels of hardness. For something a little firmer, use gymnastics grade mats. If you like a hard surface, use the mats rated for garage floors.
Cut the puzzle mats to match your couch exactly. Most puzzle mats are approximately 3 feet, 4 inches per side.
Set the puzzle mat pieces in place on your couch. Place the cushions on top of them.