Slow and steady. Offer her a small meal once every 7-10 days, and leave her alone as much as you can. Small rat should be small enough, but might want to wait until our rescue savvy members chime in.
Don't rush it though, if you try and get her to eat too much too fast she might regurge.
As far as feeding goes, I would wait a week then offer a small rat. See how she does then offer another small rat in 5 days then slowly increase rat size.