Click on the Clients page.
Search for the desired Client by using the top left "Search Clients" field. You can search by Client First Name, Middle Name, Last Name, and Client Reference.
Once you find the desired Client, click on their profile in the Clients table.
Fill in the message that you'd like to send to the Client. Once you are happy with the message, click on "Send Message".
Once the message is scheduled, you will see it appear in the Scheduled Messages feed. By default, messages are scheduled 5 minutes ahead.