Especially because you are stretching large, tight muscles, it is important to take this posture one step at a time. The form itself is not difficult - but getting your muscles, nerves and spine to open up may be.
So how do you do it? Starting from a standing position, bring your arms over your head and take a big four-foot step to your right, bringing your arms down at the same time. Then, especially if you are tight, slowly bend forward with a nice flat back. You may not be able to get your hands underneath your feet yet. No worries. Simply start by putting your hands on the floor in front of you - making sure your legs remain straight. This will allow those tight muscles to open up slowly.
Next, start working to get your hands under your heels. Eventually - before you know it - your forehead will be on the floor between your feet. Once you are there you will be stretching not only the back of your legs - you will also be opening your hips and stretching your whole spine. Then, you will want to just stay in the posture because it feels soooo good.
Benefits of this posture include opening and stretching the muscles and tendons of the legs as well as stretching your spine. It also helps the functioning of the abdominal organs. By hanging your torso upside down, you get similar benefits as you do from a headstand - without doing the headstand. Inverting this way invigorates the pituitary gland, the thyroid gland and even your eyes.
One of the best ways to learn more about the postures is to read Bikram's "Beginning Yoga Class" book. Attend class 3 to 5 times weekly to put your learning into action and make your body smile!