Hey guys,
This is my guide towards getting the full 100% sync and it's not needed to find every collectible or synchronise every viewpoint. The only thing needed for this achievement is completing the objectives for each mission. Thanks to
DarkSquall1 to point it out when it didn't know it, but seeing as I unlocked the achievement I found out myself

.
Another thing you need to know is that the game will sometimes make you free a certain amount of slaves or liberate a plantation in order to advance through the story. This will help you on your quest towards the following achievement:
Chests will give you a good amount of money, but since you receive upgrades (and near the end of the game ammo) for free, you can use that money to buy slaves and set them free or on upgrades for the Expecto Credo (which may be needed around sequence 6 or 7).
Memory I - The Calm Before The Storm- Locate the Templar Admiral's ship using the SPYGLASSWhen you gain control over Adéwalé and your ship, press

to anchor your ship and stop it completely (will help for the objective listed below). When stopped, press and hold

to look through the spyglass. Scan the 4 ships, the Admiral's ship will be the Man-O-War.
- Use the mortar to surprise the Templar FleetSet half-sail towards the ships in front of you while holding

. When the target is coloured red (instead of white), fire it with

. It doesn't matter which ship you hit to complete this objective.
Memory II - A Common EnemyNo additional objectives
Memory III - Laying The First Brick
- Perform an air assassination from a height of 10 metersWhen you gain control over Adéwalé, equip the Hidden Blades with

. Walk slightly to the right hand side and look over the edge. You should see a lone guard waiting for his death.
- Push an overseer into the waterFrom where you performed the air assassination, go to the right until you see a pond. An enemy should see you running over there. Hang on the ledge of this pond and wait for him to come close. Assassinate him and you will automatically throw him into the water.
Memory IV - A Ship of His Own- Use Bastienne's Girls to distract the guardsThe Bastienne's Girls are marked as little flowers on your mini-map, just like the Courtisans (AC: Brotherhood) or the Roma's (AC: Revelations). use them by pressing

(you don't need to pay for them) and you will see the '

'-prompt appear over guards' their heads as you move. I suggest you distract guards near the explosive barrel, since you will need it in order to advance in the story. You only need to distract one guard to complete this objective.
- Distract 4 guards with Firecrackers
Luckily, most guards are in groups of 4. To distract them, you press

. Please note that you don't throw them really far away, so make sure you can find a place to hide (bushes, haystack, on a rooftop, ...). You do not need to kill the guards to get this objective completed.
Memory V - Lifting The Veil- Kill the overseer who led the slavesUse your Eagle Sight (click

) as soon as this objective pops up and scan the area for a golden target, he should be located near the exit talking to another guard. As soon as their conversation ended, follow him and kill him.
- Liberate slaves from the slave barracksAs soon as you completed the eavesdrop on the Governor and his associate, you will need to return to Bastienne. First, head back to where you started this mission and kill the 3 guards around the slaves to complete this.
Memory VI - A Scientific Inquiry*** Spoiler - click to reveal ***WARNING: Throughout the whole mission, make sure you are unarmed so you don't kill any enemies! You will knock them out and if you don't finish them while they are moaning on the ground, you are golden
- Complete all 5 investigationsJust don't return to Bastienne when you completed 3 investigations. When you fail an investigation, you will need to reload your previous checkpoint. The investigations are the following:
* Search the group for a golden target then steal a letter
* Search the group for a golden target then follow the thief and steal his package
* Eavesdrop on scientists
* Search the area for a golden target then tackle the courier
* Liberate slaves to gain information
- Do not kill anyone while investigatingAs noted above, you will need to move through this mission unarmed to make sure you don't kill anybody. Also, DO NOT USE HUMAN SHIELD. I know this is tempting, but you are killing the guards in some way and it will not complete this objective if you do.
As mentioned in the comments below, this objective can appear rather buggy and the fact that this objective only pops up when the summary comes up doesn't help either.
DarkSquall1 's comment below might be useful as well:
For memory VI, you can do all 5 investigations without even attacking anyone. Don't even bother with fists just to avoid problems. If you get in a brawl, either escape, or reset the checkpoint. Any investigations you have completed will still be counted. For pickpocketing the key, the holder does go into an area with no other guards, so you should be able to nab it there.
Memory VII - Plant The Seeds- Sink the pirate schooners before sinking the brigAs soon as you are sailing with the Experto Credo, use your spyglass to locate the brig. It should be sailing all the way in front of the fleet. Mark it with the A-button while pressing

to see how many meters you are away from it. This way you can easily focus fire on the ones you didn't mark. The brig takes a whole lot more damage than the schooners, so if you don't focus on the brig you should be good.
- Destroy the pirate flotilla in 6 minutesThis should be easily reached if you are using mortars and heavy fire a lot. Before the Vautour took damage (it's going to happen nonetheless), I already sank 3 schooners by only using the mortars. The remaining 4 should consist of 3 low health schooners and the brig.
Memory VIII - Down with the ship- Take less then 50% damage during the naval fightVery easy to do if you combine mortars and heavy fire. Make sure to brace when your ship is under attack.
- Liberate 15 slavesThis assignment is timed once you've enter the underground of the slave ship. Slaves are mostly grouped with 3 persons each. In the first area you should be able to liberate 9 slaves, since there are only 3 groups you can release anyway. After that, and after a cutscene, you will be able to free 4 more groups of 3 (making it 12 slaves in total). Personally, I could only manage to free the first 2 groups; so make sure you don't miss during the QTE and you could be able to free the other two as well
Mission IX - de Fayet's Last Stand- Kill 5 guards with the blunderbussVery easy to do, press

on the D-Pad once to equip it. Then you can either opt to press

to shoot when an enemy is highlighted or you can use

to aim and shoot with

. If you manage to get a group of 5 together (make sure de Fayet is not in there!), you will also unlock the following achievement at the same time if you didn't already:
- Kill de Fayet with the branding ironDon't equip any weapons and when you are facing de Fayet, wait for him to attack. Counter him if he does (press

) and disarm him (press

). Afterwards you can just mash every bone and teeth out of him. Completing this objective will also unlock the following secret achievement:
Completing this memory, whether you failed an objective or not will net you the following achievement:
Thanks for reading, I hope I helped! Let me know in the comments what needs to updated or cleared up and I'll add it in if needed!

- MC0REBE