To send a private message to someone, log in to the forum and click on the username of the person you wish to contact. On their profile page, find and click on the "Activities" tab to the right of their profile picture. This will bring up the private message form where you can type your message. If you are on a mobile device, you will see the "Private Message" button as soon as you visit their profile page.

Alternately, if you already know their username, you can click on the "Messages" icon in the gray bar at the very top of your screen (to the left of your own username and picture). At the bottom of the drop-down menu, click "Send New Message". At the new message dialog, type their username in the first empty box and it should appear for you to select. Then type your subject and message in the boxes below that.