Short form announcements have 5 elements, 2 of which you’ve already provided (Title & Date/Time/Location).
The last are three sentences: 1) Describe the situation (from the audience perspective). 2) Unpack the benefit to the audience. 3) Call to Action.
Example:
East Side Car Wash
Saturday Morning 9-11AM West Parking Lot
Dirty cars are everywhere! Get your car cleaned and support East Side Students at our car wash. Come to the church Saturday between 9:00a & 11:00a!
For long form announcements (3 Before, website, FB posts, etc) you will want to take the same structure as the short form and amplify it into at least a paragraph.