Ditto on the advice that a local or nearby wildlife rescue would have some helpful tips or they may even be able to take and rear it for you until it’s old enough to be released. I volunteer at a St Louis wildlife center and they care for hundreds of baby squirrels a year. I know the really young ones like to burrow in washcloths for warmth and the Center places heat pads underneath the cages to help the babies keep warm, as they can’t regulate their body temperature very well. The younger the animal, the more times they need to be fed per day and it is a formula mixture. Good luck!!