Telling people that you are really a guy is really just a matter of preference. There are some crossplayers that prefer that people know they are crossplayers, and there are some crossplayers that like the element of stealth. In the aspect of acting female, there are a few key things that will give you away, however, as female Ranma, you have to remember that Ranma is still a guy, and will still show many masculine mannerisms. Anyway, here are some tips:
1. Try to sound less demanding and forcive in your speech.
When men talk typically, they will say "I want a Cheeseburger" and most women will say "I would like a cheeseburger." Not true for all cases, but it makes you sound more female.
2. When you sit, attempt to take up as little room as possible.
3. Try to work on your voice a bit. Nothing is more of a dead giveaway than your voice.
4. When you make the costume, try to make it complement your feminine attributes, or make up for the lack thereof.
5. A little makeup does not hurt in making your face look feminine, just don't go overboard (That tends to give you away as well)
If you need any more help, feel free to ask. Unfortunately, I have never made a female Ranma costume, so I would not know what materials would work the best, but I'm sure there are plenty that would be willing to further the help! Welcome to the crossplay world l337.