Thoroughly clean the toilet.
Add the required amount of shampoo to the toilet water, and lift both lids.
Obtain the cat and soothe him while you carry him to the bathroom.
In one smooth movement, put the cat into the toilet and close both lids. (You may need to stand on the top so he cannot escape.) CAUTION: Do not get any part of your body too close to the cat, as his paws will be reaching for anything they can find.
Flush the toilet three or four times. This provides a "power wash and rinse" which I have found to be quite effective.
Have someone open the back door and ensure that there are no people between the toilet and the garden.
Stand behind the toilet as much as you can and lift both lids quickly.
The freshly cleaned cat will rocket out of the toilet and run outside, where he will dry himself.