Action Details
Requirements and how-to articles for Steam's action:

Through Steam, you can use the announcement feature to post updates and news about your products. Your existing followers will receive these announcements through a number of channels, while new players will see them when browsing your products on Steam.

You need this to get it done:

The title of your announcement, a brief summary to grab your audience's attention (e.g. "New Update Arriving on October 8th!")
The main bulk of your announcement, where you tell your audience all the information you want them to know (e.g. features which are being added/changed in a new update, details of upcoming events and when they're happening, information about new DLC, etc.)
Before you post your announcement, you'll have the opportunity to tag it as either Workshop or Patch Notes. Tagging an announcement as 'Workshop' will cause it to show on your product's workshop hub, tagging it as 'patch notes' will decrease visibility of the update by not surfacing it to your product page.
Product Page Embed (optional)
If you link to a product page in your announcement, the link will embed with additional styling.
Embedded Image (optional)
Whilst this is optional, it is recommended to have at least one image to help grab the user's attention whilst scrolling through their activity feed/library/etc. The first image used an announcement will be used for branding in all of that announcement's thumbnails & previews. For animated GIFs, you need to upload the GIF to a popular site, and then link the URL in your announcement as an image.
Localisation (optional)
When you post an announcement, the default language will be set to English. Users with Steam set to other languages will see the English version of the announcement. It's a good idea to include localisation for all other popular languages that your players are using.
Announcement Date (Beta)
You can choose whether to publish your announcement straight away, or choose a date and time for it to appear in the future.
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How-To Articles
A list of useful links for you:

Steamworks Documentation - Community Announcements
Information about how to post community announcements and tips on how to write them.
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