Hey, took me a while to figure this out, sorry but i cant find out the very 
first picking that descends a scale, but heres the main riff the guitar does throughout 
the song!!
____________________________________
OPEN G TUNING: D-G-D-G-B-D
____________________________________
PLAYED ON A SLIDE GUITAR.
S=Slide

----0-0-0-0-0-0-0-0-0-0-0-0--S-12-12-12-12-12-12-12---|
----0-0-0-0-0-0-0-0-0-0-0-0--S-12-12-12-12-12-12-12---|
----0-0-0-0-0-0-0-0-0-0-0-0--S-12-12-12-12-12-12-12---|
----0-0-0-0-0-0-0-0-0-0-0-0--S-12-12-12-12-12-12-12---|
----0-0-0-0-0-0-0-0-0-0-0-0---------------------------|
------------------------------------------------------|

Then it just repeats, if you listen  to the song you'll see how the tab matches.
_____________________________________________________
Just play open strings then slide up to the twelfth fret.
_____________________________________________________