Well, first off, a gear clog is called a cassette.

Second, to do this right you will need a chain whip (keeps the cassette and freehub body from rotating ) and a cassette lockring tool.

third, you want to make sure the new freehub and the cassette are compatible. Usually not a problem with recent equipment but you never know.

Fourth, go here and read

http://www.parktool.com/repair_help/FAQcogs.shtml


It is not hard to do at all, but you need the right tools or you will end up damaging your parts.

Good luck arfan