Alternate between your index (i) and middle (m) fingers. Anchor your thumb on the pickup and strike the open G string with your index finger in the same fashion as with the single stroke technique. As soon as it comes to rest on the next lower string (D string) pick the open G string again, this time using your middle finger. Maintain this alternating movement between the two fingers. Make sure one of the two fingers is always resting on the D string while the other one is picking the G string and vice versa.
Always use this alternating finger style as it will give you more possibilities when changing between notes and strings.
Go back to the previous examples and use the alternating finger style.