Hello Everyone!
This can be accomplished in ACT 1 Chapter "Special Delivery." Shortly after leaving the safehouse and progressing naturally, you will come across a red flare leading to a building you can walk into. Progress a little further until you read a wall that you can climb up with a bunch of pallets stacked on top of shelves. The Golden Skull collectible is on top of one of these shelves. Please see the attached video if you prefer visual representation. Good Luck!