Turn on the tuner and lay it on a desk or counter top near you. The built-in microphone can pick up audio signals accurately from close range.
Sing a note into the tuner. When no instrument cable is plugged in to the device, it automatically defaults to the microphone setting. This is useful for tuning acoustic guitars as well as vocals.
Read the tuner's display while singing the note to measure the vocal pitch. If you are flat, sing the note slightly higher. If you are sharp, sing the note slightly lower.