Play a single note on your instrument, and tune it so that the meter needle points to the middle and the center indicatior of the tuning guide lit (if using A=440 Hz as a reference pitch). If your instrument is flat, the left indicator will light. If it issharp, the right indicatorwill light.
When I tune the 6th string in particular, it's very difficult to get it exactly in the middle for a solid green light. It goes to the sharp or flat lights also. The 5th string is a little simular as well, but I don't have the same problem for the other strings.