How to Breastfeed an Adult?
To breastfeed an adult you would need to have the women able to lactate. Then the other person would nurse on the women's breast the same as a child would to breastfeed.
You can breastfeed an adult the same way you would breastfeed a baby if you are so inclined. They can nurse at the nipple or the milk can be pumped from the breast to feed the adult from a bottle.