Our latest addition was a stray we found on a 3rd floor window sill.  When
the fireman who brought her down handed her to us, the poor baby felt like
a bag of bones.  She was estimated to be about 6 months old, and as thin as
you described your kitty.  We gave her unlimited dry food 24/7, and 4 oz.
of canned food twice a day.  That was about 3 months ago, and she is now in
perfect health.  She has filled out to a normal weight, her eyes are
bright, and her coat is shiny.  We asked the vet about nutritional
supplements, but he told us as long as she received good and plentiful
food, she should be okay.  We had her spayed about a month after we found
her, and after she had regained some strength.

feed her what ever she can handle eating. I am also a huge believer in
nutrical (or like product) I call it donuts for kitties.  There is a vet on
the board of the local shelter who hates it cause he calls it nothing but a
sugar paste, but when trying to help cats build up calories, it works.

Buy a high quality cat food too.  More nutrients less fillers so the little
one doesn't have to work so hard getting what she needs out of it.

Good  luck!  May she be in the best of health. (have you checked for fleas
and mites?)

Feed her well. My only suggestion is to have her examined by a vet. The
doctor will have some healthy feeding suggestions for your cat--they
might even prescribe a suppliment to add to your cat's food, to build
up her immune system, as well, while you're building her weight.

Best wishes,

Sneeks

Feed her a good quality *canned* kitten food and let her eat as much as she
wants.  Canned kitten food contains the most protein and least
carbohydrates- which will help her build muscle mass without bloating.  The
higher protein will also strengthen her immune system.

She should be checked by a vet as soon as possible.  Bring a stool sample
with you so the vet can check her for parasites and worms.  Strays usually
need worming.

Phil