S3ditor is much easier to use than a hex editor, unless you have a well-practiced hex brain. I used to be able to do it in the 8-bit world over 30 years ago, but this 32-bit stuff - yeesh!
For skins, anything that can read a dds file will do, and I think it was Sniper297 linked to a freeware one of those the other day... now, where was that thread?