I want to create a question that I think would be useful if added to the ones tagged faq. What should I do?
Create the question, and tag using at least faq-proposed, and support; additional tags can be added. (support is one of the tags required on the meta sites, together bug, and discussion; any question on a meta tag needs at least one of these tags.)
Other users would see the tag, and should write an answer that is as much generic as possible, without any reference to what happened in a specific case (although the question can have a reference to a specific post seen on the main site).
If the question is considered helpful for the meta FAQ, a moderator will make the question a Community Wiki, and replace faq-proposed with faq; comments for the question, and its answers will be probably removed.
Consider that, as faq-proposed is a temporary tag, it is probable it needs to be re-created; that is only possible for users with the privilege of creating new tags, which is given to users with a reputation of at least 300. If your reputation is lower, you cannot create new tags.