the best way to clean them is to replace them.

Well, you can clean them but it takes a lot of elbow grease and time.

I will let other explain on getting them out (as I think it's pointed out on the website. Probably in the technical section).

once they are out, then it's time to clean them. I do this with a scraping tool such as a small flat head screw driver, or something else that has a small flat edge that you can scrap all the carbon build up off the power valves.

Once you clean off all the carbon build up, you can use some 3m synth steel wool to clean it up, and smooth everything out. I use mineral spirits to help break things down with the scraper and wool. Clean up the inside of the cylinders of all carbon, as well as the shaft to make sure there is no carbon on any of this.

You will need to know the best time to do this if when you have the cylinders off the engine. So when you are doing a top end. You can work on the valves by removing them from the cylinders still on the engine, but to clean up the cylinders you should have them off the bike (don't want that carbon getting into the engine).
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Charles Gallant