The following example contains the G and D7 chords. When changing from G to D7, do not lift your third finger off the string, but slide it down to the second fret. Only touch the string very lightly as you do this. When changing from D7 to G, slide your third finger up to the third fret. This technique is common in guitar playing and is called using a slide finger.
Practice slowly and evenly and count or tap your foot as you play to help you keep time. There are four strums in each bar. When strumming, only your wrist should move. Do not move your arm and keep your forearm resting on the upper edge of the guitar. Remember to keep your left hand fingers just behind the fret. If you place it on top of the fret, the note will sound deadened. If you place it too far back from the fret the note will buzz and you will have to press down too hard to prevent it. If you have an acoustic guitar, pick the string over the soundhole as this results in the best sound.