Watch the Single String Approach online guitar lesson by Jason Loughlin from 1-2-3 Rockabilly Guitar
The single string approached lets us find the root of the I, IV and V chord all on one string. For this example I'm in the key of G. So, I'm starting on the G of my 6th string. You'll find the G on the 3rd fret of the 6th string. This will be the root of my I chord. Now move up two whole steps and a half step. That's 5 frets higher. We call that distance a fourth. You should find yourself at the 8th fret of the 6th string. This is our IV chord. Our V chord is always a whole step higher. That's 2 frets higher. This will land you onto the 10th fret. You can also use this approach on the 5th string. I've included a chart in the notation/tab to help you find all of your root notes.