How is a BIP accepted?
The process is not formalized precisely because only really non-controversial changes are supposed to make it in at all.

This hasn't always been the case, but is becoming more true in the last year.
Right now the process goes like this:


Start by thinking of some feature/improvement.
Convince the dev team(s) of the relevant parts (core consensus, wallets, other layers) that this is a good idea in the abstract, and it needs to be standardized.
Write a draft.
Get gmaxwell to assign a BIP number for reference.
Make everyone happy with the detailled draft.
At some point, the status of the draft changes to Accepted (based on actual agreement of all/most devs).


You can follow https://github.com/bitcoin/bips to see when changes are made.