Decarbing is a 2 part process. The first part is preventative, done by using a fuel additive to prevent carbon build up. SeaFoam is one such fuel additive and can be found at most auto parts stores. It also works as a fuel system cleaner and can be used as a preventative maintenance procedure.
If your intent is to REMOVE the existing carbon buildup from the cylinders, heads, and rings, a spray decarb solvent product, such as SeaFoam Deep Creep, is needed to do the decarb process. Free moving rings are what seals your cylinders and gives you compression. Compressed and stuck rings meanss loss of compression, broken rings, and eventually, engine failure.
1)Run engine at fast idle, with engine running and warm, slowly spray liberal amount of decarb spray into each carb. Its gonna smoke up the place - BIG TIME.
Spray for a couple minutes, now spray a larger amount into the carb(s) until engine chokes out and stops.
2)Remove spark plugs and spray the decarb product liberally into each cylinder, install the spark plugs, let it soak for an hour or more.
3)Start the engine and run at medium throttle, or if on the water, run it at full throttle. It won't hurt to spray some more through the carbs. Run it for atleast 10 minutes to flush the crud out of your engine. Now remove and clean, or better yet replace the spark plugs.
It works well to do the spraying, the night before you go on the water. This way you can let it soak overnight, and run at full throttle under load.
Don't do this in front of the garage door or the house, unless you want it covered with greasy black crud.