What is the basic exercise to do back bends in yoga?

i am a beginner in yoga. i want to make my spinal column flexible so that i can perform meditation and pranayama in better way. but i cant do backbends. is there any exercises to do backbends or what should i do to bend backwars flexibly.

I like Dances with Buddha's answer. It would also be beneficial to you to practice any pose that encourages twisting and flexibility in the spine. When doing ANY pose, focus on lengthening the head away from the body, as in lengthening the spine. This helps to encourage space between the vertebrae and will help with also encouraging the spine to become more flexible. In Tadansana (mountain pose) or any other standing pose, such as Tree, shine the top of the head to the sky, lengthen the head upward. Try to do this in all poses, this focus on lengthening. Twisting poses "wring" out the spine, encouraging it to become healthier and more supple, encouraging fresh blood flow, removing stale blood from areas that get little stimulation otherwise.
And be sure to follow Dances with Buddha's suggestions, but most of all, be patient with yourself. Focus not on the goal, but on the best you can do this moment, this day. Accept your body for where it is in development and advancement, and go into the poses with that acceptance, knowing that everything you do today makes it so that next time, you will have more strength and more flexibility. Don't look ahead trying to be there, accept what you can do today.

Good luck!

I would recommend you frequently practice asanas such as
- Bhugjangasana (Cobra);
- Dhanurasana (Bow);
- Salambhasana (Locust);
- Urdhva Mukha Svanasana (Upward Dog).

In all these asanas, especially Bhugjangasana, emphasize pushing the upper chest forward (as if the heart would burst out through the front ribs).
This stretches the thoracic region of the spine.

Often beginners only emphasize the backbend in only the lumbar region.

Also, to protect your spine, it would be wise to supplement/balance the above asanas with those that develop core (abdominal) strength, such as Adho Mukha Svanasana (Plank), and Paripurna Navasana (Boat).
Surya Namaskar (Sun Salutation) is very beneficial, and has built-in benefit of including Urdhva Mukha Svanasana .

You can also seek a teacher for instruction in the Bandhas.

Namaste',
dwb