Yeah, everyone claims to have heard rumors from a friend of a friend that it is very simple and easy, yet when you ask concrete proof from them, they always seem to come empty.
Simple dll replacement, which may possibly need you to first legitemately run steam to get a decrypted executable. The dll file may need replacement every now and then, usually with steam updates, but the basic idea is the same. Executable queries steam, hijack the function, return true, executable runs.
As to how easy it is, I'll refer you to arctic0ne's release of Civ V, which singlehandily delayed "Scene" releases of Civilization V by about 3 days. Private user, applying proper tools, then uploading the torrent.
The problem? He could upload with a maximum of 50 kb/s, thus it took about 60 hours to fully upload the game. If his release worked, he would have beaten the scene, thus no scene release could take credit for first crack. When the torrent was finally downloaded,some people could run it, some couldn't. Two hours later there was a scene release. So yes, it is (or was) as easy as "download game, replace dll, play".