how does one fire harden wood?

i've read much of fire hardening wood - especially when the making of spears is discussed - but i don't kow how it's done. can someone give me a step by step detailed account? can only "new", ie, unseasoned, wood be fire hardened? does one harden the entirety of a spear, or just the point, and why or why not? are there different methods? are some woods incapable of hardening, and others hard enough, without? thanks!