It's easy to do.
First, you must be a member and be logged in. Then go to your My Profile page.
Second, find the person who you'd like to send a message to by searching your friends list and go to their profile page.
Third, click the 'Send Message' button and then it's just like writing an email!
Note: Anytime you see a members username of Avatar (their profile photo) you can click on them and be sent directly to the members 'My Profile' page. So if a member asks a question in the forum, or posts a cool photo, or has a similar plant as you (or for any other reason), you can easily send them a message!