Hello Everyone!
This can be accomplished in ACT 2 Chapter "Grave Danger." Shortly after leaving the safehouse proceed to the right path through the cemetery hugging the right side wall. Eventually you will see an additional plot to the cemetery with multiple graves and a small stone structure towards the back. The Golden Skull collectible is in the left corner on the ground, it may be hard to see since it is in the dark and a little foggy. Please see the attached video if you prefer visual representation. Good Luck!