Here are the following steps.
The cultist looks like this:
Craft the Vampire Offering
Set up the Altar, you need 1 Summoning Altar and 8 Summoning Pedestal.
Add Vampire Offering on top of each Summoning Pedestal
Add the Summoning runic on top of the Summoning Altar
When you’ve added the Summoning Runic, a message will pop up. If you press confirm the boss will spawn and if you cancel you will need to break the runic and place it again for it to work.
Good luck!