Watch the 5th String Approach online guitar lesson by Jason Loughlin from 1-2-3 Rockabilly Guitar
The 5th string approach has us starting with our I chord's root on the 5th string. So in the key of G we will find root on the tenth fret of the 5th string. Now we're going to look for the closest IV chord. You'll find this by moving your I chord root down a whole step(2 frets) and then moving straight up to the 6th string. You should end up on the eighth fret of 6th string. The V chord is a whole step higher(2 frets). So your V chord will be on the tenth fret of the 6th string.