This achievement occurs during Episode 3 - Citizen Brown
Some time into the level after you're inside the town do the following:
• Walk over to the soup shop on the west side of town.
• Interact with Sign. (Sign below the counter, not menu sign.)
• Interact with Window.
• Dialog (Leech): "Free samples?"
• Dialog (Leech): "Never mind."
• Walk into the back alley.
• Interact with Ammo Box.
Use Item "Spray Pain" to equip.
• Interact with "Wall" (With Spray Paint equipped.)
Credit to T_l_M_B_0 of PST