Before saying shemoneh esrei we say "Hashem, sefatai siftach u'fi yagid tehilasecha". Are you supposed to say those words

A) Before you take the 3 steps back to start your shemoneh esrei B) One word per step (3 words for the 3 steps back, 3 more for the 3 steps forward) C) After you've taken the 3 steps back, before you take the 3 steps forward D) after you've taken all 6 steps (ie immediately before you begin shemoneh esrei)

I have had different people tell me some variation of these 4 options. I'd like to know which is authoritative.