You cook a sweet potato in the microwave by washing the tuber thoroughly, patting it dry, and piercing the skin several times with a fork to let steam escape. Place the pierced sweet potato on a microwave safe plate and cook on high power for five to eight minutes, turning it over halfway through the cooking time. Larger sweet potatoes may require an extra two or three minutes until the flesh feels completely soft. Wrap the cooked potato in foil for five minutes to let it finish steaming before eating.