Besides the folded Tee Shirt. or almost any kind of rag, Park makes a tool specifically for that purpose. (Rags are cheaper.)
If it is a FreeWheel and not a cassette, it is not a good idea to submerge it into a cleaning solution which will get into the bearings, dilute the grease and ruin the FW.
Stein makes a grease injector for FW's, but it ain't cheap either.