Watch the Marco Cohen Rides Again online guitar lesson by David Hamburger from Essentials: Fingerstyle Blues
If you’re looking at the notation/tab, it may look like your thumb is being asked to do a lot: not just keep the alternating bass going but also contribute the occasional melody note or low-end lick. But no matter what, the thumb is consistently alternating bass strings and picking on the downbeats. The more complex sound comes from the fact that we’re occasionally changing up the left hand, adding in a fretted note in between two open string bass notes, hammering on after picking a bass note, and so on. Take it a bar or two at a time and work through things slowly. Oh, wait, did I say that already?