You don't even need to try. Most likely she is already pregnant when you got her. They usually all are. I don't mind taking care of fry, but that's just me. I'd say to get a breeder net and have the mom give birth in there, and keep the babies in there until they are big enough to swim with the adults. The thing with platys is that they are really easy to breed, and really fun to watch the fry grow up.
EDIT: Can I keep just female platys together? Or will they fight?