General rules of thumb re: monitor output entering mics resulting in (undesired) feedback:
- position mic to face away at 90 degrees from monitors
- turn down mic gain and/or monitor volume
- eqs (or filters) can help tame trouble frequencies
- place mic behind monitors/speakers

A bit of feedback can be a very powerful tool, as in above video example!

“You must have chaos within you to give birth to a dancing star.”
― Friedrich Nietzsche