To post an announcement which will be visible to all users in your system, log in as an administrator and you will see the corresponding menu shortcut, on the top of your screen, located on the right of the navbar (1). Click on the 'Announcements' (2).
This will send you to the Announcements page, which contains a tabular list of all announcements that you have already posted in your system.
To add a new announcement click on the 'Add Announcement' button (3), which will open a pop-up window with details about your announcement.
Fill in the title (mandatory), select the Color (4) of the announcement and body fields. You can set the announcement to appear before, after, or before and after users sign in, by changing the Display (5) and then click add to create it. If you wish for this announcement to only be available for just a few branches and not system-wide, click to set the branch ownership of the announcement.
As soon as you post your announcement, it will be available for your users on their dashboard.