Brad's Iyengar Yoga Notebook
Baddhakonasana is an excellent asana for developing hip flexibility.   Also the opening conferred to the inner ligaments of the knees helps to great extent in preparing to do Padmasana.   The name Baddhakonasana means "bound angle pose," but what "angle" is "bound?"   In any konasana, "angle pose," the "angle" is the angle between your legs when your feet are wide apart such as exists in Trikonasana ("three angle pose") or Upavistha Konasana ("seated angle pose").   When you then bring your feet together, you close that angle off or "bind" it.   Hence the name "bound angle pose."
Draw your heels toward your perineum as much as possible.   Do not try to take your knees very low until you have established your heels in close to your perineum.   That is the first step.   You would like it if your calves could touch your inner thighs.   If your knees are higher than your groins, you need to take more height under your sitting bones by sitting on folded blankets or a sandbag.   Move your tailbone into your pelvis.   Notice that in this pose you are sitting more on your outer buttock bones.   You are not sitting on the same part of the buttock bones that you sit on as in Dandasana and many other poses like Sukhasana where you take your buttock flesh out and back so that you sit more on the inner aspects of the sitting bones.
Make sure you lift your trunk upward well, you can use your back against a wall at the beginning.   Establish Tadasana in your torso.   Open your chest well.   Lift your sternum toward the ceiling.   The classical pose is with your hands grasping your feet, but it's easier to establish a good lift in your torso if you take your hands behind your hips and press with them on the floor or blankets.   Once you have your torso lifting strongly, then you can take your hands into the classical position on the feet.   When your hands are on your feet, a strong grip will help give you leverage to lift your torso higher.  If your arms are short, and you cannot lift your chest well with your hands on your feet, grasp a short length of belt under your feet in both hands and pull upward on the belt to allow a good lifting action in your chest.
Manually revolve your calf and thigh muscles upward if they limit the flexion of your knees.   Stretch outward through your knees.   Pressing your heels together helps this action.   Having the feeling of moving your knees outward will also help them to move downward.   Lengthen from your inner groins outward through your knees.   Both of your knees should be equal distance from the floor.   Pull back strongly from your knees into your outer hips.   Also pull the top of your femur bones in toward your hip sockets.   As your thighs move toward the floor, it's OK if your inner feet and inner heels begin to separate, but you should keep the outer edges of your feet pressing together throughout the pose.   Try to keep your abdomen soft and your inner thighs will release more toward the floor.   Also if you are going to press your knees somewhat actively toward the floor, it helps have blankets underneath your thighs to press into and your groins will not tighten as much.   This is also safer for your groins.
One good way to make progress in this pose is to work with a wooden block.
Begin by sitting with your back against a wall with your buttocks as close to the wall as possible.   Place a block on its flat side in front of you with a mat covering it.   Put your feet up on the block, sole to sole, and take your knees gently toward the floor.   You may use your hands to press down lightly on your thighs, also turning them outward slightly (i.e. toward the wall behind you).   When this block height feels comfortable, turn the block up on its side and repeat.   Finally turn the block up on its end and repeat.   As you do this exercise, try to take your back upward along the wall as much as possible.
The next exercise begins the same way as the last, with your back against the wall.   This time, place the block on the floor in front of you on its side and place the sole of each foot on either side of the block (so the block is between your feet).   Take your knees again gently toward the floor.   When you are able to do this comfortably, turn the block flat on the floor to separate your feet more and repeat.   Finally, turn the block so that your feet are on the ends of the block, maximally separated, and repeat.
After these exercises, repeat Baddhakonasana without the use of the block and feel how much release you have gained in your hips and groins from the block exercises.
To perform this asana mainly as a restorative pose (Supta Baddhakonasana), lie back onto a bolster placed lengthwise under your torso (still keeping your sitting bones on the mat or a sandbag).   Place a folded blanket under your head and cervical spine in a way that keeps your forehead higher than your chin.   You might also place a rolled blanket under each knee for support.