first riff - after the opening, it's the same riff, but you can tag an open E onto the end of the phrase. 
Listen to the song for rhythm and repetition of the B, but I find that it's easiest just to improvise for the most part.

G ------------------    -------------------
D ------------------    ------------------- 
A 0--2--0---0-22222- or -------------------   
E --------0---------    5--7--5--0--5-77777

rest, then go back through the original riff 2x before hitting the chorus:

G -0-2-4-2h4-2-0---0--
D ---------------4----
A --------------------
E --------------------

G -0h2-0---0h2-------
D -------4-----4-2---
A -------------------
E -------------------

loop back to the opening riff:

G ------------------    -------------------
D ------------------    ------------------- 
A 0--2--0---0-22222- or -------------------  6x  
E --------0---------    5--7--5--0--5-77777


then hit the chorus again:

G -0-2-4-2h4-2-0---0--
D ---------------4----
A --------------------
E --------------------

G -0h2-0---0h2-------
D -------4-----4-2---
A -------------------
E -------------------

then, go on to:

G --5555-5555--4444-4444--12121212--7777--9999-9999--
D ----------------------------------------------------
A ----------------------------------------------------
E ----------------------------------------------------

To be honest, I'm pretty sure that's all guitar. You'll hear the bass kick back in with this:

G ---------------
D -55555---------
A ---------------
E ---------------

Then mack on the opening riff to the end.