How to be normal
1. Stay Calm.
2. Ask her if you can take a picture with her. It can start with "Excuse me", or "Hi".
3. Have someone take a picture of you two, or you can just take a picture of her, provided that she gave the consent. Don't snap stolen shots.
4. Thank her after taking the picture.
5. If you want to start a conversation, make sure that she's not occupied or no other people wants to take a picture too at the moment (else you are hogging the girl of her time).
POINTERS (If you have a different mindset)
+ Think of her as a normal girl wearing a nice dress.
+ Avoid using words with double meanings like "job" (e.g. Nice job you have there. [You are pertaining to her costume. She might think of other things] )
I recommend you to post more at
Incoherent Babbling section to further hone your talking-to-stranger skills.