I got a rescue horse that is skin and bones, what do you suggest that i feed her to fatten her up?

i got a rescue horse that is skin and bones, what do you suggest i use to fatten her up she has really badly brittle hooves also. She scared me the first time I looked at her, i get alot of rescues but she is the worst that i have ever gotten, i dont want her to founder or to colic. I just need to know what to do to fatten her up a little at a time.