Lie down on your back take a deep breath in and let it all out. Inhale and bend your knees into your chest and take hold of the outer edges of your feet with your knees at your armpits. Pull the feet down so that the knees come closer to the floor. Relax the shoulders and head on the floor. This is Happy Baby/ Ananda Balasana.
If you want to move deeper you may try to bring your arms behind your lower back , your knees will go behind your arms/shoulders and you may bring the palms together at the heart center in prayer position or eventually clasp your hands together behind your back. Keep working to expand the chest and open the hips at the same time.
Eventually you may cross the legs behind the head (right foot first) and extend the arms over the thighs and clasp the hands behind the back. This pose is called Yogi's Sleep or Yoganidrasana. The pillow that you rest your head upon here is the leg or foot behind your head. The face should be relaxed as should be the rest of you as if you were dreaming as David Swenson and Guruji would say.  
Physical benefits: Happy baby is a gentle but deep hip opener. It stretches the hamstrings and low back and groin muscles. It also helps to massage the organs, the spine and the muscles of the back.  
Spiritual Benefits: This is a pose of surrender and release. It is prone and yet held. Learning to trust and go with the breath will bring deeper release.  The more relaxed you can be in this pose deeper you will go.