Join the Companions in Whiterun and complete their questline until you reach the quest “Glory of the Dead”
During “Glory of the Dead,” collect a Glenmoril Witch Head
Go to Ysgramor’s Tomb in Whiterun Hold
Use the Flame of the Harbinger to start the cure ritual
Activate the flame with a Glenmoril Witch Head in your inventory
Defeat your werewolf form’s spirit
Repeat the ritual with another Glenmoril Witch Head if you want to cure more than one Companion
If you are a vampire lord instead, use the appropriate vampire cure method separately