Technique of mudra-
First, sit on the floor in any asana. Now put your right hand on the knee and raise to the left hand in front of the mouth but keep the face of the palm downwards. Put your thumb on the upper lip. (Stabilize the same hand by joining with the small finger and concentrate on the empty place.) (Join the same hand with the small finger Join this hand with the small finger and stable it put on the empty place.)
This mudra is very difficult in doing but regular practice makes it easy.