Lift up the cushion as far as you can to expose the stitching that secures the cushion to the top of the fabric for the main portion of the sofa.
Cut a thread used to stitch the cushion to the couch. Fingernail clippers work well for this because you can use the edge of the cutter to cut the thread.
Pull the thread out with a pair of tweezers until you remove enough of the thread that you can pull it with your fingers. Continue to do this as you work your way around the cushion.