you have to write to the secretary of state where the certificate was issued. You also need to get a recent copy of the certificate so that the same person that signed it is still around.
Type in Apostille and Secretary and the state you were married in (or born in,
whichever) into Google and you wil find more information.
the apostille is just a piece of paper which is another kind of certification that italy requires. The secretary of state in your state will also charge a fee to do this, from about 10 bucks to 25 is what i paid.
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