What you need is a straight backcast so that any forward movement will bring the fly forward. To accomplish this, lower the rod tip almost to the surface. You are going to make a side cast, so tilt the reel so it is at 45 degrees. On the backcast, use only the forearm to bring the rod tip and the line straight away from the target and up an imaginary ramp, making sure the rod remains below the head until it is beyond the body. Now the rod can stop while it is rising, and a straight backcast will occur.