Just tell the staff there is going to be a mock fire drill, and that every one is to comply.
It is a simple message, and you do not need others to write it for you. Practice writing memos and emails. If you are going to work in HR you need to know how to do it. Your company is not paying us to do your work.
With the fire drill it is important not to reveal time and date, these things need to be a suprise to work effectively. If staff know ahead of time, they will find reasons to disappear and not participate. They need to know the procedures as it could save their life one day.