Guile | Flusterstorm

Cards

({U} magic symbol) plus enough mana to cast the instant or sorcery card available.

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.
Repeat from step 3.