Pay attention that the riff has the shuffle rhythm. It means that instead of playing each chord flatly one after another with eighth notes (2 chords per metronome's click), now you use eighth-note triplets:
(It would be 3 chords per metronome's click), but you should skip each 2nd chord in a triplet.
Sorry for pour explanation, It's kind of hard to explain shuffle rhythm, though it's quite intuitive to feel and with little practice play it:
Another version of the 12 bar blues riff:
Note that in this variant of the riff each chord begins at the end of the previous bar and continue to sound at the beginning of the current one (the chords are connected with the curved lines on the tab).