Swing is created by the seam pointing consistently in one direction while one side of the ball stays smoother than the other. Most players who cannot swing it are either wobbling the seam at release or bowling too fast for the ball to move. Hold the seam upright and angled towards where you want the ball to finish, keep your wrist behind the ball, and give it time in the air. Then work on the batting side of the skill too, because a bowler who understands swing plays it far better.