The fastest and easiest way would be to use Advanced Custom Fields Pro (ACF) to create a custom metabox for your announcement. They have a resources page with lots of examples for you to pick from, and even more recipes available from community contributions. Tuts Plus has a great tutorial for getting started with ACF.
In my case, I also created an “Enable/Disable” button to turn the ACF field off, when I didn’t want it. So, on pages where I need a Google Map, I enable the ACF field for Google Maps, and enter my mapping coordinates into a list of fields.
ACF has a “builder” to help you create all the fields and conditions you need. This includes being able to specify pages/posts/CPTs to display your content, as well as users/roles permitted to modify it. There is also an exporter, which provides all the code you need to add features into your theme.
I am not affiliated in any way with ACF, nor any company, relative, or friends related to ACF and/or its affiliates.
I have built tons of customizations for dozens of web sites using only ACF. It has saved me LOADS of hours and headaches.