oh my! I think the thing to try might be to confine her to a small area, such as one room or even a crate for awhile so that she learns that the litterbox is where she needs to "go". It may sound mean but better learn early than have a bigger problem on your hands. You can still let her out for supervised play and love times but keep her confined when you are unable to watch her - until she gets used yo using her box.
If you are able to actually catch her as she begins to go potty in the wrong place - you can growl or make some sort of loud noise and immediately bring her to her litter pan and then heap on the praise when she uses it:-)
Others may have some other suggestions!