Yarsa Gumba in most cases eaten roughly. Sometimes it is mixed with milk. The most traditional use of Yarsa Gumba is it's tea. Now many Hotels and restaurants in Nepal serve yarsa tea in their menu. You can also buy commercially packaged tea bags of Yarsa Gumba.
Many Chinese and some Nepalese medicine use extracts of Yarsa Gumba in it. In order to get the benifit of Yarsa Gumba you can just take a yarsa and eat with milk or hot water.. there is no special way. So whenever you come to Nepal, I suggest you, try to eat this phenomenal gift of the nature, gift of the Himalayas.