The Commencement achievement is vague. The description only says "it has begun." This seemed to unlock for me when I completed a side-quest during this chapter. Not sure side-quest is accurate. It may be required to progress. Anyway, in the video below I start the side quest at 17:45. I take a short detour exploring, but get to the area you need to gather the items at 22:30 and finish the quest up. The achievement unlocked shortly after giving the items to the quest-giver.