Make sure you have planned out the content of your announcement before attempting to create it. You should also have in mind the post date and expiration date. Once you have planned out your announcement follow these steps to post it:
Make sure you are logged into an account that has permission to post announcements.
From the Control Panel, select the Site Content tab.
Find and select the Announcements module.
Click New Announcement in the Main Menu across the top.
Fill out the subject, summary, and other parts as neccessary.
Take a moment to proof read your content and double check your settings.
Click Save to save your annoucement.
The announcement will be posted on the date specified in Post On.