Let’s start by getting into open G (D G D G B D) from standard tuning. Lower both E strings a step to D and lower your fifth string from A to G. Strings two, three, and four stay the same. This is an important thing to remember because it means that everything you already know in standard tuning on those three strings—chord shapes, scale patterns—still applies. The basic I, IV, V chord progression in G is easy to find (Example 1) just by strumming the open G chord and barring the fifth fret for the C chord (IV) and the seventh fret for the D chord (V).
But after playing chords this way for a while you’ll probably welcome some different voicings. Down near the nut you’ll find a couple of cool chords. Putting your fingers on the fingerboard as if you were playing a regular C-chord shape in standard tuning yields as powerful a chord as you’ll ever hear (Example 2a). This puts the flatted third of the G scale (the seventh of C7) in the bass, laying some serious hot sauce on that chord grip. The D7 shown in Example 2b isn’t quite as gnarly; the seventh on the second string blends into the chord a little more, and the D notes on the top and bottom give it a nice ringing, droney sound.