How to fatten up this stray?

By the way,this little black cat in this pic is a stray who is slowly taking to me,and she is losing weight very quickly. Im trying to "fatten her up"...any suggestions?
 

She looks very sweet, and like my Elsa, who was a feral I tamed. I fed Elsa kitten food because it has a lot of calories. Although it's possible this little girl has worms.

Welcome to TCS! Thanks for caring for this little one. :heart2:

If you've been providing her food and she's still losing weight, is there any way you can get her to a vet?

Kitten food is a good choice. You can also mix in an egg (raw) for a calorie/nutrition boost into some canned kitten food. You can also offer her some kitten glop - easy to digest and high calorie/high fat: http://www.kitten-rescue.com/what_to_feed.html