Practice #18-#19 in Essential Elements, be careful not to rush ahead.

Practice all on page 9, making sure to make a “tunnel” with your left hand when there is a bracket underneath the staff to allow for the A string to ring out.

Play through exercises on page 10, and pay special attention to songs that use both strings (A and D.)  Remember to move both your left-hand fingers and your plucking finger to the new string when it changes.