The PQ system is easy. To access the PQs, click on "Private Quacks" in the user sidebar:
... or on "quacks" in the User Control Panel menu topbar:
You'll be taken to your PQ inbox:
Here you can view the PQ conversations you have ongoing, view the PQs you have sent, or write a new PQ.
If you click on the "subject" of one of the PQs, you'll be able to read that conversation. PQs which are replies of one another are sorted into one page, so you can review the entire "thread" of that communication. This means you don't have to save a ton of separate PQs!
Messages you've sent to others are kept in the "sent" box, so you can always keep track of what messages you've sent.
Please do not use the PQ system to send unsolicited updates/advertisements/spam about your comic to others. If they WANT that stuff, then fine, but just sending it to random people or anyone who has ever left you a comment is pretty rude.
BBCode is supported in PQs. For more help on BBCode, see this page.