Watch the Travis Picking online guitar lesson by Joe Dalton from 100 Twang Thangs
Learn the basic pattern with your thumb. Practice alternating between the strings. The pattern will change with the chord name. We usually pick the root of the chord first. For instance, on an A chord you would pick the fifth string open A, but on an E chord you would pick the sixth string open first. So learn the pattern for the specific chord. After you are comfortable with this you can start adding melody notes in a simple rhythm. After this you can add syncopated notes.