Hi Nkansah,
thanks for the message.
We achieve that through the CentralNotice feature available for Meta
administrators <http://meta.wikimedia.org/wiki/CentralNotice>. You can
read all about this notice system, and also see the currently running
campaigns at <http://meta.wikimedia.org/wiki/Special:CentralNotice>
(marked with a green background).
Setting up a CentralNotice campaign does not require too much work
once you learn a bit about the software behind it. Unfortunately for
the planning Wikimedia chapter for Ghana (but probably more
fortunately for the readers of Wikipedia), the usage of CentralNotice
is rather limited, only to important notices for the readers of the
projects or for the community itself.
The usage guidelines for CentralNotice are available at
<http://meta.wikimedia.org/wiki/CentralNotice/Usage_guidelines>.
Hope that helps!
Tomasz