From Yoga Journal:
Traditionally Fish Pose is performed with the legs in Padmasana. Since Padmasana is beyond the capacity of most beginning students, here we’ll work either with the knees bent, feet on the floor, or with the legs straight pressed against the floor.
Step by Step
Lie on your back on the floor with your knees bent, feet on the floor. Inhale, lift your pelvis slightly off the floor, and slide your hands, palms down, below your buttocks. Then rest your buttocks on the backs of your hands (and don’t lift them off your hands as you perform this pose). Be sure to tuck your forearms and elbows up close to the sides of your torso.
Inhale and press your forearms and elbows firmly against the floor. Next press your scapulas into your back and, with an inhale, lift your upper torso and head away from the floor. Then release your head back onto the floor. Depending on how high you arch your back and lift your chest, either the back of your head or its crown will rest on the floor. There should be a minimal amount of weight on your head to avoid crunching your neck. (For more about this, see the Beginners Tip below.)
You can keep your knees bent or straighten your legs out onto the floor. If you do the latter, keep your thighs active, and press out through the heels.
Stay for 15 to 30 seconds, breathing smoothly. With an exhalation lower your torso and head to the floor. Draw your thighs up into your belly and squeeze.
Benefits of Gate Pose
The backbending position in Matsyasana can be difficult for beginning students. Perform the pose with your back supported on a thickly rolled blanket. Be sure your head rests comfortably on the floor and your throat is soft.
Beginners sometimes strain their neck in this pose. If you feel any discomfort in your neck or throat, either lower your chest slightly toward the floor, or put a thickly folded blanket under the back of your head.
Matsyasana, fish pose, stimulates the 5th chakra, located in the throat. Associated with the color blue and named for purity in the form of wisdom or Visuddha, the throat chakra is the body's gateway to spirituality, expression, compassion, and will. Too often neglected, this chakra bridges the divide between our inner and outer world. It shapes our circumstances and creates the conditions that manifest in our lives. Caring for the throat chakra promotes clarity, creativity, vitality, and balance, while blockages bring miscommunication, frustration, impatience, and addiction.
From: http://www.togetheryoga.net/2/ post/2012/03/asana-focus-of-the-month-matsyasana-fish-pose1.html