You choose which permanent to sacrifice as you make choices for the spell (see rule 601.2b), and you sacrifice that permanent as you pay the total cost (see rule 601.2h).
July 13, 2013 · Magic 2014
The card with ninjutsu remains revealed from the time the ability is announced until the ability leaves the stack.
September 30, 2011 · Innistrad
A player can't cast spells once a spell with epic he or she controls resolves, but effects (such as the epic ability itself) can still put copies of spells onto the stack.
June 17, 2011 · Commander 2011
A player can’t cast spells once a spell with epic he or she controls resolves, but effects
February 1, 2010 · Worldwake
Multiple instances of convoke on the same spell are redundant.