Complete the Companions questline until you can access the cure quest
Speak to Farkas or Vilkas after finishing the Companions storyline
Obtain a Glenmoril Witch Head during the quest “Purity”
Travel to Ysgramor’s Tomb
Activate the Flame of the Harbinger
Use the witch head in the flame to summon and defeat the werewolf spirit
Repeat the process for each cursed Companion if curing multiple people
If you are a vampire lord instead, cure that separately before curing lycanthropy if needed