Go to Settings > Email Newsletter > Send Sample.
Yes - it's that simple!
An important note: The intent of the Send Me A Sample feature is centered on showing you the layout of your newsletter, not the content selection logic. Because of this, sample newsletters often show content that may not make sense to you. For example, Top Stories may not appear at the top of a newsletter in the sample. Also, the sample newsletter goes back 180 days when looking for posts, so there may be cases where you posted 10 'Top Stories' in the past 180 days and your sample only shows Top Stories. Don't be alarmed by this, as this won't be indicative of your live newsletter!