What I tell beginning guitar students for learning how to use the thumb on alternate beats is to: finger a C chord. Now, with your right hand play the A string with your thumb, next play the B string with your middle finger, Play the A string with your thumb again, then, play the G string with you index finger. That's a simple Travis picking kind of pattern. Just repeat at a speed you're comfortable with, and you can speed up gradually. When you get tired to the C chord, change to another chord and figure out what strings you want to play in that pattern. I, too, never took lessons. I have taught them. The exercise was something I read in Sing Out, written out by Michael Cooney a long time ago, but he should still get the credit.