Hello, Phoebe! Do you mean ascending or descending? One note or multiples? When I do a one-note descending gliss, I keep my hand in a fist and use the thumb. For ascending, I use the 2nd finger with the other fingers in the palm and the thumb up. You could, alternately, leave the fingers open and pointing towards the column in an ascending gliss, or leave the hand open with the fingers pointing up in a descending gliss. Try all of the above and listen to the resulting sounds, and also notice how your hands feel.