Hello Everyone!
This can be accomplished in ACT 1 Chapter "Book Worms." Shortly after leaving the safehouse and progressing naturally, you will come across a giant green shamrock or clover sign. Progress a little further through an alleyway to the left hand side of the level where you have to go for the main campaign objective. Once you blow up the propane tanks, head through the house and up the stairs, immediately turn left and the Golden Skull collectible is behind the office desk that has some copper on top of it. Please see the attached video if you prefer visual representation. Good Luck!