Watch the The I-V-vi-V-IV-V Desc. Bass online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
We'll extend the previous descending bass line progression by repeating the I-V movement before going to the IV, and on that repeat, instead of playing the I again we'll use its relative minor, the vi. We'll once again use versions of each chord without necessarily putting the root in the bass in order to create the step-wise descending bass line. In order to make the second V chord work in a descending bass line, in the key of G playing a D with an F# in the bass by hooking your thumb over, in the key of D play A with your first finger and add a C# with your third finger, and in the key of C play a G with a B as your bass note played with your first finger. For the second V chord, I might hint the chord instead of playing it by staying on the previous chord, the vi, but put the root of the V chord in the bass. For example, in the key of C, I might play an Am with my fourth finger on a G on the sixth string.
Examples: Piano Man, Smiling Face, Tears In Heaven, Dust in the Wind