Feed breast milk first, and when there is not enough supplement it with formula. In the beginning, feeding the baby as much formula they want after they have been breastfed should be fine.
For example, if you are feeding the baby every three hours, breast feed them at 6 in the morning, then formula at 9, then at noon breast milk, then at 3 formula, alternating your feedings.
In general, the amount of milk produced in the breasts declines in the evening, so with this method, you breastfeed only in the morning, then alternate between formula and breastfeeding in the afternoon.