tips for getting baby back on breast.

Any tips on how to get my baby back on the breast. She's four days old and because of his small she was at birth and wasn't latching properly and my milk wasn't in I was advised to give her formula. I've been pumping lots and giving her the expressed. I've also got nipple shealds which she seems to latch better with. I'm just worried she won't get enough milk and don't know how often and how long I should be feeding her.