This is the first time I draft a proposal, so I'm not quite familiar with the related procedures even after reading
How To Submit a Proposal : Standard C++ (isocpp.org). It seems that before presenting the proposal to LEWG, I need to present it first to SG16 and the paper needs to be assigned with an official proposal number. But I'm not quite sure what I should really do next, so I send a mail to SG16 mailing list. Any help would be appreciated :-)