How to make an OC and NOT Mary Sue

The title is pretty self explanatory. Just know, I am NOT a complete expert. I feel as if some of my OCs are Mary Sues, if not, just very overused. I just know some stuff is very basic and you can be super creative if you try to! :)

Btw, the fandoms I'll be covering the most are creepypasta, undertale, and warrior cats, but really this goes for any ocs.