1) Navigate in the Finder to /System/Library/CoreServices/
2) Open "CoreTypes.bundle" by right-clicking it and selecting "Sow Package Contents" from the centext menu, then open the "Contents" folder, then "Resources"
3) There they are! Find a suitable icon (preview either with "Get Info" or by double-clicking, which will open them in Preview)
4) In a second window, navigate to the folder you want to decorate.
5) Select it and open its file info with "Get Info" (either using Command + I, the context menu, or "Get Info" in the "File" menu)
6) Drag and drop the .icns file onto the little icon at the top left. (Not onto the big preview.) You have now successfully copied the icns resources.
To revert to the default icon, just select the small icon in the file info and hit delete.