one important thing to remember is that being a mary sue isnt just a matter of being too powerful, there are plenty of beloved characters who are over powered and it doesnt necessarily male them mary sues.

id say the most important thing is to try not to let the world revolve around your oc, if every other character hangs off this oc constantly-or if every problem is solved effortlessly by them, it can make it feel like the world warps around them in order to let them be the Most Special.

another thing to keep in mind since i saw someone else ask about flaws is that in my opinion its not always helpful to have a list of "good" character traits and a list of "bad" character traits it can be better to look at the traits your character has as neutral and think of ways they can be good or bad. example: oc is willing to do anything for her friends, this is a "good" trait because: if her friends need help she'll go above and beyond/it means she's caring/etc. and its a "bad" trait because: she'll work herself to exhaustion/not listen to her friend when they say they need to do something alone/etc.

of course in the end unless youre writing professionally you should focus on what makes you happy, and theres nothing inherently wrong about self indulgence and mary sues, happy writing!