There is no better way that I know of screwing up a file beyond all chance of recovery that by getting a hex edit wrong. But if you want to do it, the best advice I can give you is to always and without any exception whatsoever keep a backup file. But if you want to do this, on your own head be it.Basically, what you need to do is to go through the savegame files slowly and carefully, and try to identify correspondences between the hex values that you find, and the decimal values that appear in the savegame files. Also, search through the back posts for "hex". IIRC, Klar posted some useful data here.
But do take care, and always keep a backup. Good luck! 
