Make sure the kitten is kept warm.
Make sure the kitten is toileting well and not constipated. Some times they need enemas to clear the bowel.
If it goes off its feed seek help as they quickly become dehydrated.
If it inhales milk, they may need antibiotics to make sure they do not get pneumonia.
If their bottom gets sore, use some sort of ointment to prevent nappy rash.
Weigh daily with a scales accurate to the g to assess progress.
HAND REARING KITTENS - a good link.
Good luck.