scrape top layer of cd w/ anything abrasive. If it's super confidential you should probably shred it afterwards, otherwise unless the CIA or someone with high-end equipment gets it and wants to read (parts of) it you should be rather safe at that point.
For simple jobs I use perm.marker on the clear side (bottom) but I imagine some sanding and polishing could defeat that. So, again, if it's highly confidential and risky, then if you scrape away the top layer (reflective stuff) you are good. And follow the bad directions, srape off in circles not straight lines from inner to outer.