I can tell you, this is pretty hard to do by oneself.
If you are looking to replace system icons, such as folders, they are here:
If you want to replace icons for apps under /Applications, it has to be done individually. Typically:
After replacing it, the new icon will not show until the icon cache is reset. In Mavericks, resetting LaunchServices would update the icon, but in Yosemite, from the time I spent with it, "iconcache" has to be deleted.
All in all, it takes patience.