To introduce formula to a baby just mix up some formula in a bottle and usually if the baby is hungry the baby will eventually take to the formula.
But if you're able to breastfeed then the breast milk is best for your baby.
But sometimes we can't produce enough breast milk and the baby needs to drink formula. It may take awhile for your baby to get used to the bottle nipple if your baby has not been bottle fed at all but he or she will eventually take to the bottles nipple.
You just have to keep trying.