By the user ticking the box labelled "Community wiki" at the bottom of the answer form.
Several years ago it also used to be the case that a large number of edits (I'm not sure of exactly how many, maybe a couple dozen) would forcibly convert your answer to a wiki answer. This no longer happens, but old posts aren't switched back automatically (though they could be by a mod.)
Every post, both questions and answers, has a "share" link beneath it. Copy what it gives you to have a direct link to the individual post.