You will win this Achievement after completing Sequences 1 and 2.
You will be in control of Haytham Kenway throughout this section. You do not require 100% synchronisation for this.
Play through the opening level in London, enjoy the sailing trip to Boston, meet your 5 contacts.
The end of sequence 2 comes when you and your men disguise yourselves as Redcoats and free some slaves from a Redcoats named Silas. After freeing the slaves you have to kill Silas. Once this is complete, Achievement Unlocked. Congratulations.