Sit on the mattress with your legs stretched out in front.
Bend your legs at the knees and bring them under you.
Sit on your heels.
Slowly flatten your feet, parting them slightly at the ankles, till your buttocks are resting on the underside of your feet.
Hold this pose for as long as possible - which will probably be just a few seconds the first time. Try again after a few minutes, and sit in this position for at least one minute. Slowly increase the duration everyday, till you can sit comfortably for half an hour in this position. It is excellent for digestion, so if you have any problems with gas, you should definitely give Vajrasan a shot.
Don't do this Aasan if you suffer from heart ailments or any leg/ankle injury, unless you are supervised by a yogic guruji.