How do I prepare aging cedar siding for staining/painting?
If it's real cedar, it shouldn't be more complicated than scraping, sanding, and restaining.  You'll have to sand down to bare wood before restaining.  Depending on what kind of siding you have, it might be hard to sand it everywhere, in which case painting it might be a better option.  For paint, you just have to make sure you scrape/sand to the point where the paint has a good surface to adhere to.  Then prime and paint.

But you should make sure you know what you have.  Real wood is less and less common as siding, so you should make sure you don't have something like Hardiplank.