For riffs which involve playing notes on all four strings a new pattern must be used. In this pattern each riff position starts on the 4th string, as illustrated here:
Try the following riff.
Play this riff as a 12 bar in G, starting at the 3rd fret for the G ( I ) chord, the 8th fret for the C ( IV ) chord and the 10th fret for the D (V) chord.