To cause feedback, move the instrument (your mouth, a guitar or anything that produces sound) closer to the microphone. If feedback does not occur, turn the microphone volume up on the corresponding channel on the mixer. To decrease feedback, move away from the microphone and/or decrease the volume.
If you're still getting feedback, slowly turn the master volume control down.
When the feedback disappears, find the volume control for the specific microphone that caused the feedback. Raise all the volume back up except this one.
Slowly increase the volume on the channel of the microphone that initially caused the feedback. If undesired feedback occurs again, lower the volume slowly.