There is a wonderfully informative thread on making fermented feed but it is terribly long.
I will summarize for you.
Take regular chicken feed (layer or all flock)
add water until the feed is covered by 1/2 to 1 inch of water
add Apple Cider Vinegar (the kind that is unpasteurized like Braggs brand) 1/2 Tablespoon to 1 Tablespoon per gallon of water
It will expand greatly so use a container that is much taller than the feed you are putting in.
I started with a quart jar filling it only half way. I then graduated to using frosting buckets I got from the gorcery store.(they give them away if you ask at the bakery)
Stir and allow to sit over night and up to 3 days. You may need to add additional water if the mix is very dry. It will need to breathe so cover with plastic wrap with a hole cut in the top or drill holes in the bucket lid for air. It will also have an odor (kind of unpleasant if you ask me) The longer it sits the stronger the odor.
I feed it as more of a wet mash the next morning after making it. My chickens went crazy for it right away.
I would feed 1 cup to the skinny hen where she has NO competition for food two times a day. If she cannot finish it all decrease the amount you give to reduce waste. She should be able to get a good amount in her within 20 to 30 minutes. I locked my other chickens in the run so mine could eat in peace. I also found a frisbee from the local
petsmart that cost me only a dollar worked very very well as a dish.
I am sure there are many options for fermented feed. This is just what helped my old skinny hen.