It's important with a kitten like this to make sure she has been seen by a vet to check for any problems related to the malnutrition and to ensure she has been properly wormed. Assuming she is underweight but healthy there are two big considerations as regards getting her feeding right.
1. Transitioning - the kitten will be being fed a food by the rescue centre. If you want to change this then you should do so gradually, taking at least 7-10 days to do so. I'd further suggest keeping her on the diet fed by the rescue centre for the first week after you bring her home as the changes in her environment can lead to tummy upsets.
2. It sounds obvious but you do need to be feeding a kitten food. Growth isn't complete until around 12 months and a kitten food will be higher in energy and nutrients that your kitten really needs. Check and see what kind of food she is accustomed to (canned or dry) and feed her a format that you know she likes. You can introduce other formats once she is settled on her new food. I'd suggest you feed something made using high quality ingredients which will be easier to digest. Hill's Science Plan Kitten is a good option and is available in pouch, can or dry formats from pet-supermarket by clicking here.
By Libby Sheridan (Hills Pet Nutrition) (Professional Advisor)