Announce to the congregation a week or two before the drill of your intent to conduct an emergency drill. Tell them when the drill will take place, and what is expected of them. This is especially important if this will be the first drill. Unannounced drills should only be performed once the congregation is comfortable with what to do when the alarm sounds.
When a congregation is beginning the practice of holding drills, it is best to schedule 3-4 drills in the first year. After the congregation is comfortable with the procedure, yearly drills are generally sufficient. Holding regular drills helps familiarize occupants with the sound of the alarm system and allows your congregation to practice evacuation procedures, so in the event of a real emergency, you have rehearsed the procedures and are aware of what to do.
Expect the first drill to be chaotic. The more the congregation practices, the calmer and more organized they will be.