You have several options depending on the length of time since you last breastfed and your normal milk production quantity. If it hasnt been long you maybe able to just let your baby suckle at the breast and bring your milk back in. If it has been some time you can start to pump and drink mother milk tea.
If you have not breast fed for a while it will be difficult to get your milk supply back up and going. But you can attempt to use a breast pump throughout the day for this. Every time your baby wants to eat, you should be pumping. Make sure you pump both breasts equally.
Let the baby suck on the breast briefly before giving it the bottle of formula. This may be more successful if the infant is not desperately hungry. The stimulation of the sucking causes the hormones to kick back in produces lactation. Pumping can also help.
The more milk you remove from breast the more milk it will produce. For restarting breast milk feeding, start feeding the child four to five times a day even if the milk is very little. Gradually quantity of breast milk will increase. You can use pump, which will stimulate the breast resulting in producing more milk.