1. To create them, just go to Manage/Communication/Home page messages. There you'll find an "Add a message button" - click it!
2. Fill in the name and content of the message, you can also add the background image. Then choose the action after clicking. You can also set the dates, so that the message will be shown at certain period. When you're done, just click "Save" and then "Publish".
3. Next step is choosing the recipients of the message. Just go to either "Recipients" or "Recipient groups" tab and add users.
4. When you're done, the message will be shown for chosen users at the top of their home page.