I have a simple exercise for this: Get four players to stand 2v2 facing each other about 3 yards apart. One side are attackers the other defenders. The attackers stay still but give one of them the ball and make the defender facing the player with the ball move forward so she is right in front of her to "press". Players stay still and you explain this is pressing with the second player staying back to cover. Now get the attacker to pass the ball to the attacker next to her. Now the first defender drops back 3 yards to cover as the second defender moves right up to "press" the attacker with the ball. Do this a few times to show the defenders what it is they should do when the ball is in play.