Sit on the edge of a table. Place a sling/band around the ankle of the leg you want to stretch.
Grasp your other knee, pull it close to your chest and roll onto your back. Relax the leg you are stretching, let your hip straighten and knee bend. Increase the front thigh stretch by bending your knee further with the sling/band.
Note: Don't let your hip bend or the arch of your lower back increase.