SLY MASK #1
Head to the extreme north of the map to find a shuriken on the side of the road. Hop on it, and use Riochi's Dragon Leap to cross the gap. A mask lies on the ledge on the other side.
SLY MASK #2
Now head to the extreme west of the map to find another shuriken. Once again, Dragon Leap across the gap to find a mask on the platform on the other side.
SLY MASK #3
During the "Breakout!" mission, proceed until you encounter Riochi in the cage. Just past this is the wall you must crawl under. In the left corner is another spot you can crawl through - it leads to a mask in a cell.
SLY MASK #4
Also during the "Breakout!" mission, play until you pass the second set of fire-breathing dragon statues. Just after, you'll reach this sleeping guard. Hop the gap to the right to find another crawl spot. Head through to find another mask in another cell.
SLY MASK #5
At the beginning of the "Sushi House Startup" mission, you'll learn how to use the Dragon Leap technique. Instead of leaping to the fan, look toward the entrance to spot another ledge you can reach. There's a mask there too.
SLY MASK #6
At the end of the same mission, climb the pole on the side of the boiler room to find another mask.
SLY MASK #7
"Something's Fishy" and "Spiked Sushi" have you in the fish caverns. Turn around upon entering to find a mask behind you.
SLY MASK #8
Play through "Pretty in Pinker" until you reach the spinning fans with the rats patrolling below. You can bust the box in the corner of the room to reveal a mask.
SLY MASK #9
Proceed through "Altitude Sickness" until you enter the dragon gate. Immediately after the cut scene, look right to find this mask on a ledge.
SLY MASK #10
During the same mission, you'll cross a burning bridge while pursuing El Jefe. On the other side, look left of the steps to spot a mask tucked into the corner.
SLY MASK #11
Once you've recovered every treasure in the level, the arcade machine in the hideout will be repaired. Boot it up to play Alter Ego, a side-scrolling shoot-em-up game. At the end of the first zone, you'll have a choice to go right to a enter portal or further down into the zone. Enter the portal to reach a zone full of purple globs. The world's final mask is in this zone.
Note: For more detail on collecting the treasures required of this mask, see our Sly Cooper: Thieves in Time treasure locations guide.