After speaking with Samuel, you will head to the harbor where you have to kill 15 soldiers. They are all marked on the map with red dots. Fighting them isn't hard, but remember to cover yourself from shooters by pressing the jump button.
Optional objective: Dump crates of tea in the water - 10
Optional objective: Throw redcoats in the water - 3
Optional objective: Perform musket air assassinations.
Optional objective: Completed all the constraints in one playthrough.
With the last enemy killed, the second part of the mission will begin. You will have to protect your companions from the enemies until the tea is thrown into the water. This time you will also receive some additional objectives. Apart from fighting the British soldiers, you will have to dump 10 tea crates into the water. Do it at the very beginning before the enemies get to you. The boxes can be picked up with the interaction button and thrown with the jump button.
Your second objective will be throwing three British soldiers into the water. In order to do that, attack the soldiers who are standing at the shore. It's best to counter them and press the interaction button again during slow motion so that Connor throws the enemy.
The last mission is a bit harder. You need to perform an aerial attack while holding an enemy musket. The problem is that you can't climb onto a mast or ropes while holding the weapon.
To perform an aerial attack, you should stand onboard a ship and attack a soldier running onto it.
Afterwards help your companions throw the rest of the tea. That way you will end the mission.