This stretch can be done kneeling on the floor or kneeling on the edge of the bed. It doesn't matter which one.
Start with your hips sitting back on your heels then lean your upper body down towards the floor/bed.
While you're resting on your elbows in this position, you're going to use your elbows to walk your upper body around to the side. Just the forearms and elbows should be doing the work. You should feel a stretch on the opposite side of your low back.
Use your hands, elbows and forearms to walk around the other side until you feel a stretch on the opposite side of your lower back. Hold that stretch for five to ten seconds, whatever feels good, and do it about five times on each side.