According to Michelle Ruberg in the Writer's Digest Handbook of Magazine Article Writing, there are three rules to writing an as-told-to article: first, you must use the first person to get the intended emotional effect. Don't interject your own thoughts into the article. You must become the subject when your write the piece. Second, act as both writer and editor. Put the subject's account into chronological order and make sure that it all makes sense. Third, before having the article published, ask the subject show the piece to the subject for final approval. That way there are no surprises when the story is published with the subject's name underneath it (ex. "by Suzy Subject as told to William Writer").
Where do you find ideas for an as-told-to article? Michelle Ruberg suggests opening up your local newspaper. A local resident might be mentioned briefly in a column and you realize there's a story to be fleshed out there. Contact the person mentioned and ask them if they would be willing to share their story. Who knows where it will lead?
Who publishes as-told-to articles? Michelle Ruberg points out that Guideposts for Kids often publishes this type of story. Women's magazines like to focus on as-told-to articles about woman who have accomplished something or overcome an obstacle. Prevention magazine has included several as-told-to pieces shared by Amy Purtel's blog at http://www.amypaturel.com/articles/list/category/7/ctitle/As-Told-To/.