This song is played with a capo on the 7th fret.
The song is essentially a continual repeating
of the picking pattern shown below, between 2
chords (D and A7/C#, although you play these
by forming the "G" and "D7/F#" chord shapes
relative to the capo).

It's up to you how you choose to pick the
pattern, although I've watched Justin play
this and for the G chord he frets the top
string with his ring finger, using his pinkie
to fret the notes on the bottom 3 strings
when he needs to.  This makes it easy to
change into the second chord.  When he
changes into the second chord, he places
his middle finger on the second fret of
the top string and uses index and ring
finger to do the other fretting on the
2nd and 3rd strings.

As for your right hand, alternately pick
the bass notes (i.e. the repeating "E"
and "d" strings) with your thumb and pick
the rest with either your index finger or
a combination of fingers.  Whatever feels
comfortable!

This is quite a difficult song to master
at first, especially if you haven't
practised much independent finger picking
before.  But once you can play it, it's
really fun and sounds great!

"G" (D)

e ----------------|
b ----------------|
g ----------------|
d --0---0---0---0-|
a ----------------|
E 3---3---3---3---|

"G" (D)

e 3-3---3-0-----0-3p0-------------|
b ------------3---------0-----0---|
g --------------------3---0-------|
d --0---0---0---0---0---0---0---0-|
a --------------------------------|
E 3---3---3---3---3---3---3---3---|



"D7/F#" (A7/C#)   "G" (D)

e --------------------------------|
b ----1---0-----0-----------------|
g 2-----------2---0---------------|
d --0---0---0---0---0---0---0---0-|
a --------------------------------|
e 2---2---2---2---3---3---3---3---|

(repeat this section once before starting again with the previous section)