Feed her a high calorie cat food for nursing cats, and mainly she needs to drink lots of water to produce milk, you can improve her water intake by also feeding her canned food:)
What Do I Feed A Mother Cat To Produce Enough Milk?
A higher fat diet...and extra fluid as in water. And you can even feed her kitten food (my veterinarian approved that).
Just feed her , her normal diet. Her body will produce milk naturally .