I like to move into this pose from three-legged dog, but you can also move into this from a tabletop position. From three-legged dog, bend the lifted leg at the knee and bring that ankle to the opposite wrist (i.e. – if the right leg is lifted, bend the right knee. Bring the right knee to the right hand and the right ankle to the left wrist so the leg is (as) perpendicular to the body as is comfortable for you. This front ankle should now be in line with the opposite hip. Keep this front foot flexed and begin to lower down to the mat, while sliding the back leg straight back and sinking the hips to the floor. Only lower down as long as you can keep the hips square. Keep the hands on your mat near your hips and sit up as straight as possible, keeping the spine long. You can also take a forward fold over your tucked front leg by sliding the arms forward and hinging at the hips. For more of a challenge in pigeon, you can take your arms off of the ground and add more of a backbend to the position, or even move into mermaid or one-legged king pigeon pose.
*See Reclined Hip Opener for another option that stretches the hips without placing pressure on the knee joint!