How do you make a spirit weapon with the spirit stone on Mabinogi

Make sure you're at least lvl 26 and you don't have Eiry.

Assuming you did the first part of the quest, (if you didn't, you can search it up on the Mabinogi Wiki):
Get your weapon to 100prof, 30 elemental removers (1k each; can be obtained from Lassar, Price, or Galvin), and the desired spirit stone (if your weapon is a sword, then you need to get a sword spirit stone, etc).

Go to Tarlach in Sidhe Sneachta who appears around 6PM-6AM at night. He's the only NPC who can form a spirit contract.