How:  In all content (posts, status updates, blogs, etc.) you should use the report post link for anything which needs to be brought to the attention of the mods.  It goes without saying that on a forum with this much traffic it's completely unrealistic to expect the mods to read every post; hitting the report post link will send them a notification so they can deal with the situation.  Clicking that link will give you an optional field to send a message with your report, but that's not required.

What:  Free Jinger is a largely self-policing community, which is why it's important that everyone does their part to make sure the few rules we do have are being followed.  That means reporting when you see someone violating a rule or ToU such as doxing, opining on sexuality of minors or adult children still living at home, speculating on unnamed victims, hate speech, etc.

If a violation has been made and not yet addressed by a mod or admin then report it - it's better to get multiple reports than for everyone to assume someone else reported it and for this stuff to go unchecked.  If you aren't sure whether something is a violation or not then report it and let the mods make the call.

You can also use the reporting function to alert mods to threads they may wish to know about due to quickly escalating contentiousness as an FYI.

The reporting feature is not to tell on people who disagree with you, thread drift on a topic which bores you, swearing, or people speaking in a harsher manner than you'd like while clearly within the bounds of FJ culture.  Communication on FJ is generally pretty direct and not everyone will love that.

There wouldn't be enough time in the day to deal with reports if every disagreement or critique that wasn't delivered with a back-rub and a gentle kiss on the head were reported to the mods.  And FJ wouldn't be able to function without self-policing members and reports when warranted, so use your judgement.