Hold as many cards as you are going to change, tip between your 2nd and 3rd finger not letting the audience see them. this can be done by distracting the audience with patter and gliding cards to the left. Put the cards in your right hand (the ones to be changed) in between the left 3rd and 4th finger acting like straightening out the cards while taking out the other cards that are in between your 2nd and 3rd finger.