Sometimes a song does not begin on the first beat of the bar. Any notes which come before the first full bar are called lead-in notes (or pick-up notes). When lead-in notes are used, the last bar is also incomplete. The notes in the lead-in and the notes in the last bar add up to one full bar. When you are playing chords, do not strum until the first full bar after the lead-in notes.
Practice each of the chord changes and then play the whole song. When changing between the F and G7 chords, use your first finger as a pivot. When changing between G7 and C, slide your third finger along the first string.
Rhythm Pattern