Hello.
In order to target a
SPELL, that spell has to be on the stack. This means that it has been cast, but it has not resolved yet. It will go something like this:
1) Your opponent casts an instant or sorcery spell targeting you. (Spell goes on the stack)
2) In response, you cast
Mirror Sheen targeting that spell. (
Mirror Sheen goes on the stack)
3) (Assuming no other spells/abilities on stack)
Mirror Sheen resolves. You will create a copy of the spell that your opponent cast. The copy will be created on the stack. You will get to choose a new target for the spell. You are the controller of the copy.
4) The copy resolves.
5) The original spell resolves.
Hope this helps.