If she is a rescue (which would explain the age and training level) Then you're going to have to show her how to use a doggy door, but given that she is super small (teacups don't exist except the the tea world
, btw) you shouldn't let her out alone. Try from square one, like you are training a brand new puppy, limit her space and provide pee pads on a hard tile/wood floor. Praise praise praise when she goes in the right spot. It can be difficult but really rewarding, don't get frustrated, it'll probably take a while before she is fully understanding of what you want her to do.
Good luck! Look in the training section of the forum for some really good advice