This stretch was named and made popular by Kelly Starrett of MobilityWOD. It’s a simple stretch to do, and it’s great at hitting the hip flexor and quad.
It’s called the “couch stretch” because you need a couch to do it, but a wall or something similar will also do. You may want something soft to put under your knee, though, depending on your floor surface. You’ll see in the video I also use a chair in front of me to support myself.
Experiment with what works for you, but remember never to force things or push into a “bad” pain. See if you can do five minutes on each side at an intensity that is tolerable — and make sure you keep breathing deeply the whole time!