I'd say to practice the communication during workouts. DTS near the catchers feet, he quickly comes to his feet, rakes the ball in, looks to 1st hollering "outside". Ball gets out toward batters box, catchers should move toward the ball, rake it in while continuing momentum onto infield grass hollering "inside". In both cases it should be practiced so that the catcher has a clear throwing lane where he can fire the ball. Remember that the 1B must also practice on setting up correctly, similar to a bunt situation, I prefer the 1B not to stretch with his throwing side foot, but get the inside throw with his glove side foot, and outside throw with his arm side foot. He can face directly at the catcher, in the event of a bad throw he can still jump or move side to side. By the way, very important, what age group?