Step 1: Materials
Equipment necessery will be:
A rear cluster from a bicycle. I got mine from the back of a bicycle store in a big bin full of metal.
A Dremel or other spinning cutting tool with a nice skinny blade.
Degreaser / barbecue cleaner. Grease on your hands for the rest of your day may just cause female relatives to faint.
Safety specs. To protect your peepers from flying cutting discs.
Gloves. Not necessery but useful for keeping your hands free from what the degreaser does not get. I didn't use them, and paid dearly for it.
A screwdriver for a bit of leverage where necessary and changing the cutting disc on your Dremel.
A hacksaw. Makes finishing off the last bit of cutting so much easier.
A vice to hold it when cutting. Not necessary but definitely useful.