You arrived in the city earlier today with the lady of the house and her husband? And you made sure they both agreed that Charlotte should come along to help the lady of the house get ready so she would look as beautiful as possible?
Do not refuse to dance when a gentleman asks you; it's considered very rude in the 19th century.
Be polite at all times (as difficult as that may be).
Avoid talking too much or laughing too loud.
Don't get snarky with anyone.
And remember: the guests will be removing their masks at midnight, so Charlotte will need to leave before then. Yeah, I know, shades of Cinderella. I'll be on pins and needles until I hear from you.