1 – The mask is hidden in Ozan's farm. You must jump into the well and dive into the pool of blood.
2, 3 – Two masks are inside the main building of the farm where the Beelzebufo boss can be found.
4 – Mr Volkan is wearing the mask and the Crash Boom Bang side quest is about finding him. Locate Volkan's body after you've completed that quest.
5. 6, 7, 8 – There are four masks inside Atilla's mansion. You visit this location for the first time while completing the Vanitas main quest. One of the masks is lying on the desk and the remaining three are hanging on the walls in different areas of the building.
9 – The mask is by the altar located to the south-west from Atilla's mansion.
10 – The mask is inside a house located on the other side of the street than the gas station. It's hanging on the bedroom wall on the upper floor.
11 – The mask is by the altar located in the crossroads. There's a mummified corpse here and the mask is attached to its face.
12 – The mask is near the altar, right next to a tree.
13 – The mask is close to an altar. You'll find it on the rocks that are forming a small tower.
14 – The mask is by the altar found on a small hill. It's on the ground right next to a mummified corpse.
15 – The mask is located near the altar with the mummified corpse. You'll find it lying on the ground.
16 – The mask is on top of a large rock located next to the Temple of the Sun. You can reach the temple yourself or you can wait until you're doing An Unfinished Story main quest which is about exploring the temple. Once you're close to the temple you can either climb on the other rocks and jump to the one with the collectible or use a grappling hook.
17 – The mask is near the altar located on a hill close to the dam. You'll find it right next to a mummified corpse.
18 – The mask is by the altar. It's put on a mummified corpse found in this location.
19 – The mask is inside a house. You must go to the upper floor and you'll find it hanging on one of the walls.
20 – The mask is on a small island on the lake. You'll find it attached to the rocks next to the altar.