Prerequisites:
1. Have completed at least
Sequence 9.
2. Have at least
57 Sync Points (complete main story and co-op missions to get these - if you complete all story and co-op missions, and collect all the extra sync points in the co-op missions you will have enough points to buy EVERY skill, so you shouldn't worry about purchasing the "wrong" skills)
3. Purchase the following skills:
Melee (22)
* Long Weapon Master (4)
* One-Handed Weapon Master (4)
* Staggering Strike (2)
* Heavy Weapon Master (4)
* Ground Execution (8)
Ranged (11)
* Ranged Weapon (1)
* Assassin Cache (10)
Stealth (24)
* Disguise (6)
* Apprentice Locksmith (4)
* Roll Recovery (2)
* Double Air Assassination (4)
* Double Assassination (4)
* Environment Blending (4)
4. Then go into the Café Théâtre. You
may need to have completed the following restorations to unlock training (7,250F):
* Ground Floor - 250F
* Hall - 500F
* Exteriors - 1,500F
* Upper Floors - 5,000F
Once done, go to the second floor, and by the stairs is the training room. Go up to the guy sitting on the bench, and hit

to select a training mission. If the guy is missing, then see the
NOTE about other ways of accessing the training)
Go through all
21 training missions once to get a checkbox by them.
The achievement will pop when you complete the final training mission.
IMPORTANT: if the achievement doesn't pop after you've completed all the training, then you will have to repeat them all in one session. This may be related to when you started playing the game and doing the training. If pre-patch 4 (16th Dec 2014 - the 40GB one) you probably will have to do it in one session.
NOTE: You can also complete skills training two other ways:
- From the Customize option (Press

for the map, and then

when over Arno) > Skills > Select a skill and press

for Training
- From the Menu (

) > Database > Tutorials > Training