If you receive any new messages, a popup window will appear if you visit any InsaneJournal page (excluding users' journals, etc.). It will say "You have a new private message. Do you want to view it now?". From there, you can click "OK" to automatically go to [http://www.insanejournal.com/privatemessages.bml].
Any messages you receive will appear and be stored at [http://www.insanejournal.com/privatemessages.bml] until you delete them. From here, you can see if the messages are New (marked with an "X"), who they're From, the Subject, and the date when it was Sent to you.
All Free Patient and Early Inmate account users will have a storing limit of 100 messages. Self-committed [paid] and Permanently Insane users will have a storing limit of 500 messages with mailbox support. For information on purchasing or upgrading your account, see [http://www.insanejournal.com/support/faqbrowse.bml?faqid=8].
To send a message, click on the "Send Message" hyperlink. The top drop-down box labeled as "Select Below" conveniently lists all users on your friends list. You may also type in the recepient's username in the field below. Community accounts cannot receive or send private messages.
The Subject field is optional. When the user receives your message, the subject you have entered will appear. The subject field has a limit of 255 characters.
The box for the Body is where you will type your message. There is a limit of 1,000 characters.