The Announcments forum appears in the introductory section of every Moodle class.

To post a message to students, open the forum and click the Add a new topic button

Compose a discussion post just like in a normal discussion forum, making sure to include a short, clear subject - for reasons that will be clear in a minute.

Click the Post to forum button to send your message to your students.

Students have three ways to see your Announcements posts. First, they can visit the forum in the class and read the posts there. Second, they will see recent posts listed in the Latest Announcements block on the right side. Keeping your message subject brief and informative will make it easy for students to scan in this block.

Third, because subscription to the Announcements forum is forced, all your students will automatically received new posts as messages forwarded to the email associated with their Moodle account.