Hands to Feet pose comes immediately after doing Half Moon pose and a backward bend. The combination of these postures opens the spine by moving it in ALL 4 directions. The idea is to elongate the spine outward through the crown of your head as you fold into the pose.
As a beginner, let your knees bend as much as necessary to grasp your heels from behind with your fingers pointing toward the mirror, elbows BEHIND the calf muscles, keeping your feet together.
If you can't reach your heels, don't be discouraged; you're not alone. Just reach your hands to the back of your calves, or as low on the back of your legs as possible. This is better than putting your hands under your feet from the sides.
As you lay your body against your legs, bend your knees even further if you have to. If you can’t get your body to touch your legs, then just do your best to touch your forehead to your knees. Unfortunately, nothing will accomplish the ultimate straightening of your legs except day-in, day-out determination and much strength. Try concentrating on lifting the hips rather than on the straightening. At the beginning you will feel pain! But this kind of pain is normal, expected and good - a step beyond discomfort.
The goal is to keep upper body and lower body SANDWICHED together through the entire posture. Once your body is able to touch your legs, you want your upper body to be completely flat against your lower body as much as possible. Bring your torso to the front of your thighs FIRST by straightening your back and lifting your hips. THEN try to reach the top of your head towards your toes.
Pulling is the object of the stretching! Pull on your heels as hard as you can and LOCK YOUR KNEE. "Locking the knee" means tightening your quadricep muscle as hard as possible so that the knee naturally lifts.
See how much further you can get your elbows behind your legs. See if you can make them disappear so they can't be seen from the front.
Stretch your upper body down by lifting your hips high and sucking your stomach in. This will help to straighten your back and legs. Stretch your face to your knees, eventually your smiling relaxed face will go below your knees, and the top of your head will touch the top of your toes.