Watch the A La James: 2 online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
Okay, so first tune your low E down to D. We're going to use our left hand hammer-on and pull-offs for the bass to free up our right hand.
Hammer-on all of the fretted notes and pull-off to get the open D and A.
The riff is D-low D-D-C-A-D then the chromatic walk up the 6th string from 4th fret to 7th fret F#-G-G#-A (not open A!) then C-D on the A string. That last D is the first note of the repeat.