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Begin this pose in standing in tadasana. Transfer weight to left side, find your dristi, connect to your bandhas, lift your right leg and bend at the knee and open the hip to the right. Place your right foot in the crease of your right leg and thigh. (IF this bothers your knee bring your foot over your thigh by the knee or come into tree pose). For more advanced students you may bind the right hand around your back and hold onto your right big tow in yogic toe-lock.
If the bind is not to be leave the arm at your side. You may begin to fold forward bringing the unbound hand or both hands to the floor or a block for balance. You may also bring you hands to a wall folding at your waist body parallel to the floor arms straight, if folding to the floor feels precarious. Breath five or so long and deep breaths and then come back up to stand. Release your right foot to the floor and repeat on the other side.

1. Stand in Tadasana (mountain pose)
2. Inhale bringing right leg up, placing right foot on the top of the left thigh.
3. Exhale reaching right arm down to the ground, while left arm is brought around back to hold on to the right in lotus pose. Try to keep left leg straightened, but if you can not, bend the left knee to release some pressure.
4. Stay in posture for five deep breaths (inhales and exhales).
5. Inhale looking half way up to lengthen the spine.
6. Exhale out of posture by releasing left hand from right foot, lowering right foot, taking right hand off of ground and standing back in Tadasana.
7. Repeat all steps performing pose on left side by replacing all right actions with left actions and left actions with right actions
A variation on this pose to modify it to make it easier could be to not bring left hand around back to hold right foot up, but just place both right and left hands on the ground.

Physical Benefits: This posture opens up the hips, opens up the shoulders, and stretches the hamstrings. Core muscle in the standing leg is also built because this posture is a balancing posture. Because it is a forward fold, this posture increases the blood flow to the lungs and head.
Therapeutic/Spiritual Benefits: This posture can help stimulate digestive organs and relive constipation. Also, because this posture is a balancing posture, it helps ground you and build focus.

Take great care while attempting this pose lest you cause yourself an injury.
If you are attempting this pose for the first time then do so under the supervision of an experienced instructor.
Do not attempt this pose if you have a problem with your knees.
Keep your hips drawn under you. If you cannot do this, then you may compress your sacroiliac joint.

Ardha Baddha Padmottanasana (Half bound lotus) is a one legged balance pose.
Actually, it's a one foot and one hand balance pose. The non-standing leg is in lotus position. The lotus arm grabs the lotus foot from behind the back.
The hand on the standing leg side is on the floor beside the standing foot.