Many of the songs in this book can be played using the G and D7 chords you learned in Lesson 6. You will need to be able to change from G to D7 and back again.
Strum a D7 chord twice, then change to a G chord. To make this change easy, slide your third finger along the first string to the third fret. Only touch the string very lightly as you do this.
When changing from G to D7, do not lift your third finger off the string, but slide it down to the second fret. After you have played this exercise twice, finish by strumming the G chord once.