If you're in doubt if some content violates the rules, report it, and let the mods decide if it's okay or not okay. That is not abuse of the report function.
Include a short description on why you're reporting some piece of content. Specially in larger comms, the mod queue can get really large. If reasonable mention the rule being violated; a simple "r1 off topic" goes a long way.
Context is everything. If what a user said only sounds bad in a certain context, say it. If the user is clearly problematic due to their profile, say it.
You're probably better off not interacting with the content that you're reporting.
Don't boss the other users around. It's fine to be informative; it is not fine to act as a moderator when you are not one. If moderative intervention is necessary, report it.
Stop giving shitty mods a free pass. Honest mistakes happen; but if the mod in question is assumptive, disingenuous, trigger-happy, or eager to enable certain shitty types of user, spread the word about their comm being poorly moderated. And don't interact directly with the comm. I think that at least here in the Fediverse we should demand higher standards from our mods.