How do I best clean the cog wheels?
You don't need to take the cassette (cogs) off to clean it.

Use a citrus degreaser fluid. Clean the chain and chainrings at the same time. You can use a rag pulled taut to get between the cogs, or there are a number of specialized cassette and chain cleaning tools available.

There are many instructional videos out there with tips on cassette and chain cleaning also.