This guide was written for, and uses screenshots from, Microsoft Outlook 2013. However, the steps and interfaces involved will be very similar to those of previous versions of Outlook.
To begin, go to the File menu, and click on Automatic Replies.
In the Automatic Replies window, select the 'Send automatic replies' radio button, set a timeframe for the automatic replies to be sent (if desired) and then write your replies in the editors. There is a different tab for internal emails from the same domain as your address and external emails from any email address outside your domain.
Once the replies are written as desired, click OK.