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Go into a very public place.

Walk around a while, so that people get used to your presence.

In the middle of somehwere with lots of people, suddenly throw yourself on the ground, and start screaming at your imaginary boyfriend/girlfriend. Have a huge fight with them, screaming obscene things like "why did you sleep with her" and things like that. Make sure there's noone close enough to you to look like you are screaming at a real person.

NOTE: This isn't a spell. But it will get people scared of you. Trust me.