How to write and propose an RFC?

The responses by Jörg W Mittag and Darin Dimitrov are perfectly correct but they forget to add that writing a RFC and having it adopted is a long process which requires time, dedication, a hard technical work and a lot of social interactions.

This is both an answer and a further question. Given that writing an RFC "requires time, dedication, a hard technical work and a lot of social interactions", wouldn't a good idea be to talk about it with other knowledgeable people before doing all that, possibly in vain?

I think that the correct place to do that is on the "IETF (Internet Engineering Task Force) mailing list" isn't it? (I once participated in IETF discussions amounting to say "How can I correctly write my name André?" (it was the joke at that time but the general project succeeded extremely well with UTF8 (which is just an encoding but that became the synonym of the universal character code ISO 10646 because it is unique))).

So, I wonder why the first topic of RFC 7322 et al. is not "how to start preliminary discussions (recommended)".

There's actually an RFC for that: RFC 7322 - RFC Style Guide.