If you have plastic carboy, which I'm assuming. I put a scoop of generic oxyclean (Awesome Clean/Dollar Tree Brand), and 1 gallon of warm water.
Then I use a solid carboy bung (without the hole drilled) and place it upside down in my sink or garage. Make sure the cleaner is in contact with the ring. Every couple hours I give it a shake. By the next day everything is gone.
Rinse thoroughly, sanitize and store for next use.