If someone forgot to sign a message they posted on a talk page, you can do it for them by using the {{unsigned2}} template. Simply place the template at the end of the comments, and include the user's name and, if possible, the timestamp (which you can get from the article history}. For example: {{subst:unsigned|MyNameisForgetful|21:18, 11 January 2016‎ (UTC)}} would look like this:
— Preceding unsigned comment added by MyNameisForgetful (talk • contribs) 21:18, 11 January 2016 (UTC)[reply]