Country Backup Basics
So here it is, our first bass/strum move. It’s not so different from doing four straight downstroke strums, except that the first strum is replaced with a single note, the root of the chord.
The root of a G chord is G. There’s a G an octave down from the open third string, and it’s played at the third fret of the sixth string. We've already been fretting and playing this note as part of a G chord. In Example 2, try using it in a G-chord bass/strum: play the low G on the first beat, followed by three strums.
Got all that? Let’s try a C chord. The root of a C chord is the note C. We've learned the C on the second string; there’s another C on the guitar at the third fret of the fifth string. We've actually already fretted this note, as part of playing a C chord. It is notated on the first ledger line below the staff. Those two C notes are an octave apart. If you play or sing them, you’ll hear that they are in tune with each other.
To do a bass note and strum together, play a single low C on the first beat, followed by three strums on beats 2, 3, and 4 (Example 1).
Then, try switching between a C chord and a G chord (Example 3).
Excerpted from The Acoustic Guitar Method