During memory 5 of sequence 3, you'll come across a plantation that has guards and alarm bells. The best way is using a running assassination (hold R2 and square at the same time while you have swords or hidden blades activated) on the guard.
Just so you know, getting in a fight before you kill him doesn't count and using guns doesn't count either.
Edited, thanks to Spilner1001 for pointing out my mistake