As in flash your boobs? Just pull up your shirt

1. Open trenchcoat
2. Hula hoop action
3. Run away from yoga studio

If we are talking about dance flash mob, you generally need music and one person to start. :)

With flash mobs, is better to have a large group of people who are in on it, but act normal. Gets larger faster

make sure you have a well lit studio beforehand and understand how the flash will change how the picture looks. Adjust your exposure to fit.

Im really hoping you dont mean flashing innocent bystanders. But if you mean flash mobs then usually there are online stuff for groups near you lol.