Chet Atkins was an absolute master of fingerstyle
guitar. To play his rendition of "Imagine" by John Lennon
pick the notes on the low E string and A string with your
thumb, pick the notes on the D string with your 1st finger, pluck
the notes in the G string with your 2nd finger, pluck on the B
string with your 3rd finger, and pluck on the high E string with
your 4th finger.
Drop D Tuning
This song is in drop D tuning. In drop D tuning the low E string
is tuned down one whole step to D. This enables the guitarist
to play power chords with a single finger. If your favorite band
uses drop D tuning and you want to experiment with this tuning,
by all means do so. What ever you do, don't learn the guitar in
drop D tuning-you'll get all mixed up.
To tune your guitar to drop D tuning, first make sure your tuned
to standard tuning. Then you'll want to tune your low E string
down two steps to D (we drop it down one step, hence the name).
See the picture below. You can use the audio clip below to tune
your low E string to D for drop D tuning.