How do I put an automatic reply (e.g. out-of-office message) into Outlook?

The following Microsoft guide shows you how to create and enable an automatic reply in Outlook: 
Send automatic out-of-office messages from Outlook 2007 and later (microsoft.com)

If you're using Outlook on the web (office.du.se), you can find instructions here:
Send automatic (out of office) replies in Outlook on the web (microsoft.com)

This article helped me!