The most commonly recommended formulas are goat's milk, and Kitten Milk Replacment (KMR brand). I've never had to hand rear kits, myself, but one lady I know says she was most successful using goats milk mixed with baby formula (and optionally whipped cream to make it more rich like rabbit's milk).
Use an eyedropper to give it to the baby (let him lick/suck it--don't squeeze it in). Make sure the baby is upright when feeding it so it doesn't drown. Fill to round, but don't overfeed (the mothers will usually only feed them once a day). Use a warm wet cloth on it's bottom to help it relieve itself (they can't do it on their own initially). Keep it in a warm nest box (preferably with its siblings...if not, try a heating pad on /very/ low heat...they make nestbox heaters). Warm water bottles can also be used, but you have to keep an eye on it, because if it goes cold, you'll be working backwards. You can stop giving it formula about a week after it shows itself capable of eating solid food (usually will start nibbling a few days after its eyes are open).
