Watch the Alternating Bass: 2 online guitar lesson by Muriel Anderson from 1-2-3 Fingerstyle Guitar

In general, the bass pattern usually starts on the note that is the name of the chord - the root. On a G chord or E chord, alternate between bass strings, 6 - 4 - 5 - 4. On an A chord, alternate between strings 5 - 4 - 6 - 4. On a D chord, alternate between strings 4 - 3 - 5 - 3.