This achievement is specifically tied to Chapter 1. Once you reach the mid-way point of this Chapter, Garret will mention needing to gain entry to the Jeweler's Shop. Once you have located the building and have made your way inside, you will need to find two flashy party masks scattered around the house. The first mask is located in the cellar, inside a locked safe behind a painted portrait. The combination to this safe is 7-3-9. Alternatively, the second mask is located just upstairs, within one of the locked cabinets that line the showroom. Be careful here, as there is an NPC in the cellar who will alert the guards if he spots you and the showroom has a guard patrolling about. It would be wise to take them out before attempting to loot the party masks. Either way, once both masks are retrieved, you will unlock the achievement.