Crossroads

This is just a standard 12-bar blues riff and I think it is pretty accurate. 

Riff A
G:----------------------------
D:-----7-5-7-5-7575-----
A:-----------------------7---        x1
E:---5-----------------------

Riff B
G:----7-5-7-5-7575------
D:----------------------7----
A:--7------------------------            x1
E:-----------------------------

Then play
Riff A x2
Riff B x2
Riff A x2

G:----9-7-9-7-9797-----
D:----------------------9----
A:--9------------------------    x1
E:---------------------------

Then you go back to Riff B play once
Then you go back to Riff A play twice then the 12 bars start again

Jack Bruce does some cool bass lines on the 12 fret when Clapton is doing his solo but is just improvisation so you can do anything as long as is goes with the 12-bar blues