Surprise her:
Of course every man makes it a point to keep the whole thing a secret from his girl but surprises have always not gone down too well in some cases.
You need to test the flow of water yourself first. Dropping the P-bomb out of blue might be a shock for her. So instead while you are upfront and all out there planning the entire thing. Start dropping hints to see if she is picking them in a positive way. Trust us a knew-it-all along smile is definitely better than an embarrassment.