For those who don't know, overlays are the posters, paint marks on walls, extra detail on ground, etc you see in-game. I searched for an overlay tutorial on the forums, did not find one, and searching the web found this excellent one on the Valve Developer's site. Rather than re-invent or plagiarize the wheel, here is the link: http://developer.valvesoftware.com/wiki/Hammer_Overlay_Tool It goes over the basics such as using the overlay tool and more advance things like changing what brush faces it should appear on (yes you can do multiple faces!!), wrapping, and tiling of the texture. Hope it helps.