You have a way to cast an additional instant or sorcery, or an opponent is casting one.
({UR} magic symbol) ({UR} magic symbol) ({R} magic symbol) ({R} magic symbol) , plus enough mana to cast the additional spell if needed available.
Steps
Cast any other instant and/or sorcery spell, or wait for an opponent to cast one.
Holding priority, cast Expansion by paying ({UR} magic symbol) ({UR} magic symbol) , copying the next instant or sorcery spell you cast with mana value four or less.
Cast Fork by paying ({R} magic symbol) ({R} magic symbol) , targeting the instant or sorcery spell cast in step 1.
The delayed trigger from Expansion triggers, copying Fork, with the copy targeting the original Fork.
Resolve the copy of Fork, copying Fork, with the copy targeting the original Fork.