You have an additional instant or sorcery card in hand.
({U} magic symbol) plus enough mana to cast the instant or sorcery card available.
Steps
Cast the additional instant or sorcery spell.
Holding priority, cast Flusterstorm by paying ({U} magic symbol) , targeting the instant or sorcery.
Flusterstorm's storm ability triggers, creating a copy of Flusterstorm for each spell you've cast before it this turn, with each copy targeting the original Flusterstorm.
Resolve a copy of Flusterstorm, exiling Flusterstorm and allowing you to cast it from exile without paying its mana cost, targeting the instant or sorcery spell.