You can download the prebuilt application from the link at the top of this page, or just download / fork the project and built it yourself.
After you've got the app running, you can drag single files, multiple files and directories to convert @3x or @2x images. Dragging is allowed in the Retini window aswell as the App Icon in the Dock.
The resized images will be placed in the same directory as the source file, if you drag a @3x file, a @2x and 1x will be generated.
The original file will never be altered, the @2x and 1x are copies. If you drag a @3x file, the 1x file generated will be generated from the (original) @3x.