Go to Falion in Morthal
Buy a filled Black Soul Gem
Ask Falion to cure your vampirism
Travel to the marked ritual site at dawn
Meet Falion there with the filled Black Soul Gem
Let Falion perform the cure ritual
If you are a Vampire Lord, use the same Falion cure method
If you are a Werewolf instead, cure lycanthropy through Aela’s questline before curing vampirism if needed