Remove wheels. remove caliper and support it !! dont let it hang !!. Remove caliper carrier. ( The thing the caliper bolts to) remove screw that holds disc in place if it has any and pull the disc off. If its stuck give it a crack as your not going to re use. Refitting is the opposite of what ive just said but make sure the mating faces are clean as can be as this will help things remain as true as it can be. Maybe a light smear of copper grease on the spigot before the disc goes on. Dont forget to use brake cleaner when re fitted to clean of any protective oil and crap from your hands. Double check everything as you refit then check again when yoyu think your done just for good measure. Its a doddle once youve done it for the first time