I use a terry rag stretched across the edge of a tie plate, a flat metal square about the size of a credit card that sells for a few cents at any home improvement store. I spray the cassette with degreaser and then push the plate with the cloth into the gap between the cogs. I hold the plate stationary and turn the cassette on the freewheel with the other hand...this is with the bike on a repair stand. It cleans it down to the spindle and makes the job go quickly.