I've been dancing for over four years now, and I can honestly say that I'm still not really happy with my rotary chasse. That's not to say that it's bad, but rather that I've moved on to more advanced problems than you. (FYI: I'm competing, at D grade level in all three [Modern, New Vogue, Latin American] styles.)
It takes time (a lot of it) and practice (also a lot of it) to get the rotary chasse happening nicely. That said, I'll try to explain what I believe the girl should be doing. (I'm a guy, but anyway.)
You start off facing your partner. He moves off on his left foot, you move off on your right. Step to the side, at an angle.. so if your feet start pointed directly at him (towards center), you'll end up with your feet pointed down line of dance. Bring your other (left) foot in to close, also pointing down line of dance.
After this, you step forward on your right foot, whilst the guy steps back on his left. As you come through, your body turns so that you're facing 180 degrees from when you started... and then you start the process as if you were a guy. Step to the side on the left foot, close on the right, step back on the left (again, turning as you go so you're facing back line of dance at the close).
After you've done two complete turns (four sets of "side together forward/back"), one of two things can happen. In the case of, say, Gypsy Tap, where you're in closed hold, you stay in closed hold and move off for the start of the dance. In the case of, say, Merrilyn or Charmaine, though, you need to bring your right foot in to your left, turning clockwise to face down line of dance, ready to move off. (Your arm should be raised pretty much straight up as you turn, to avoid knocking your partner out.)
This is how it's supposed to be done, not necessarily how you'll do it at first. Most of what has already been posted is a good starting point, and will get your body (even if not your feet) moving in the right general direction. Dancing is not something that you can do immediately, so what usually happens is we start by learning something that's wrong, but getting us ready to learn something that's less wrong. And so on, and so on, approaching the correct way of doing things over time.
Which class are you going to? You may be learning from somebody who is explaining it poorly, or who is explaining it well for most of the class, but not you in particular. Alternatively, you may just need to dance with somebody who's more experienced to get a feel for it. There's plenty of dances out there; for example, I go to a social dance in Cheltenham on Thursday nights (the Ritz); there's one in Ferntree Gully (also on Thursdays -- Mar Shere); East Bentleigh (Ritz - Saturday); Bentleigh (Star - Friday); Mitcham (Diamond, don't know what night); ... Try going around to several; if nothing else, the extra practice will help you get used to the steps.
Finally: don't dread the "waltzy" dances. Do as many as you can, and try to get at least the side together side going. Practice is the key.
Hope this helps,
Stuart.
dont forget to have fun swaying around and around.