How do I make my question as 'community wiki'?
I can't find the button anymore, I think there was a checkbox once.
The checkbox to make a question community wiki from the start is no longer available as a regular option for users. As @paulmorriss mentioned, you can find more details about community wiki in general here. You can flag your question for moderator attention if you truly think a question should be community wiki. Be aware, however, that more often than not, someone wanting to community-wiki a question is a hint the question doesn't belong on the site.
We have provided some additional guidance at the blog:
Most of the time, you should be asking yourself “How can I improve this post so that community wiki isn’t needed?” Community wiki is like a cheese knife: it is a specialized tool to be used sparingly, and only in very specific circumstances.