1. You can choose to send a message while viewing someone's profile by selecting the "Send a Message" button under their profile picture. This option is also available from the right side of any search results screen. You can also select "Compose" from your Inbox, and type in the name of friends you would like to message.
2. Once you call up the message box, just type a subject and a message and click "send." The person you send the message to will be notified the next time they log in.