Once you have a small group of interested people, the next step is a proper taster with a certified instructor — not a casual session led by yourself, unless you're also certified.
Basics to organise:
A space roughly 6×8m, cleared of obstacles
A speaker capable of filling the room
30–45 minutes is usually enough for a first taster
Confirm your instructor's qualifications and insurance in advance (see "What qualifications should our instructor hold?" in the Fitness Managers category)
After the session, ask directly: "Would you come every week if this ran locally?" A room full of "yes" answers is genuine evidence of demand — useful if you later want to approach a venue or funder.