Great question. First of all, this can be a difficult transition, because you are going from lots of feedback to no feedback. So ensure you can hit at least 6 consecutive solid shots with the two-ball drill. Once you eliminate the second ball: (1) address the remaining ball slightly off the toe, and (2) envision that second ball just outside of the ball you are hitting. Be very conscious of keeping the hands arms and club close to your body through the hitting area. And if you shank 2 or more balls in a row at any time, go back to the two-ball drill.