The Single string picking pattern uses a combination of thumb and index fingers on your picking hand. It alternates between thumb on the down beat, and index on the upbeat. The G major scale is first shown on one string to display the pattern in a linear fashion. It is played with only the thumb. We only use our index finger when playing the 16th notes as seen in measure 4 and on. If it's just 8th notes use the thumb. There are exceptions, but for the most part, "them's the rules"!