If you are editing an existing section, make a copy of it in a new page and edit from there. This allows you to work in a sandbox.
The Talk:User_Interface_Guidelines page should list at the top all the pages being worked on. Actual content should be on the working page or in a bug report.
Put a comment at the top of the existing page, "UNDER CONSTRUCTION bug #123" with the bug number. This is a safeguard to ensure two people don't start making major edits to the same section and potentially either duplicate or lose work.
Put a link to this page in your bug.
Have things reviewed
Change the status of the bug to resolved with resolution fixed. This will trigger review.
Wait a while. If there is violent objection someone will reopen the bug. Discussion can happen on the bug thread or UI Best Practices Working Group mailing list. Issues can also be brought up at one of the meetings.
Merge your changes into the drafts
If after one week nobody has raised any objections to the changes, move your content from the sandbox to the draft pages here: UI Best Practices v3.x