Holding your baby skin-to-skin (your baby in just a diaper on your bare chest) boosts your milk-making hormones.
@nhasx3 one way is to offer the breast for 20-30 minutes every 2-3 hours. Try to nurse on both sides. If baby is not willing to breastfeed often, use a double electric pump to express milk and stimulate milk production. Nurse before, after and between feedings for as long as the baby is willing - no matter how much formula is given.
Also your baby may become frustrated by feeding from an empty breast at first, so you could try hand expressing enough (or pumping just for a moment) to trigger milk flow before putting the baby to the breast.