The tuning is drop D (DADGBe)
Practice this to get the rhythm down
it's not that hard to catch.

e------------------------------------
B------------------------------------
G----5---5---5---4---4---4---0---0---
D------------------------------------
A--0---0---0---0---0---0---0---0-----
D------------------------------------

The Riff continues on to this:

e-------------------------------------
B-------------------------------------
G-------------------------------------
D-----(2)slide(7)--7-----7------------
A--0-----------------0-0--------------
D-------------------------------------

This is an advanced version:
(it is also the actual riff)

e----------------------------------------------
B----------------------------------------------
G----5---5---7-5-4----4---4---4---7---7---(b9)7-5
D----------------------------------------------
A--0---0---0--------0---0---0---0---0---0------
D----------------------------------------------

e-----------------------------------------------------------
B-----------------------------------------------------------
G----5---5---7-5-4--4---4---4----0----0---------------------
D---------------------------------------(2)slide(7) 7-----7-
A--0---0---0------0---0---0---0----0------------------0-0---
D-----------------------------------------------------------

Then the song gets heavy. I'm not sure if this
is it, but it sounds like it.

e-----------------------------------
B-----------------------------------
G-----------------------------------
D--0--0--0--2-3-2---0---0--0--0--5--    X 4
A--0--0--0--2-3-2---0---0--0--0--5--
D--0--0--0--2-3-2---0---0--0--0--5--


e-----------------------------------------------------------
B-----------------------------------------------------------
G-----------------------------------------------------------
D--0--0--0--2-3-2--0---0--0--0--7-6--0-0-0-2-3-2-0-0-0-0-5-6
A--0--0--0--2-3-2--0---0--0--0--7-6--0-0-0-2-3-2-0-0-0-0-5-6
D--0--0--0--2-3-2--0---0--0--0--7-6--0-0-0-2-3-2-0-0-0-0-5-6