RIFF 1 - You're gonna do this one alot, with some minor variations on how many times the notes are played.
G-|------------------------------------|
D-|------------------------------------|
A-|-33333333---------------------------|
E-|----------3333333-5555555-----------|

RIFF 2 - Play when the music changes, you'll hear it.

G-|------------------------------------|
D-|------------------------------------|
A-|-33333333---------------------------|
E-|----------33333333-555555555--------|

There are some other little runs the bassist does, but they're not essential to the song.  This is the main part.  It's the way I've always played it, and it sounds good.