For this you have to be able to Recruit Assassins, which starts in Sequence 4, and I suggest you atleast get 5 since its easier to complete contracts, and its one of the Guild Challengs (rank 5 to Assasins). Go to the Pigeon Coops or Assasins Tower to get a contract, and always pick the highest exp as you can, they need to get to rank 10, or 3500 exp so it might take awhile. You're able to send out multiple assassins on the same mission which will increase your success rate.
After that and it says you have to go to Tiber Hideout to attend the ceremony, if you dont know where it is, go to a tunnel entrance and go to the Hideout, then exit the Hideout through one of the doors, than reenter. You will have a cutscene and after that and a loading screen achievement will pop.