Navigate to the Webmail Home screen by following the steps outlined in the Accessing Webmail article.
Once on the webmail home, scroll down until you see the Autoresponders item
Click to access the Autoresponder settings
Create a new autoresponder by clicking on the Add Autoresponder button.
Fill in the details on the form that is presented:
This should be fairly self explanatory. If you want to set start and end date/times, click on the appropriate custom radio button and you'll be presented with a calendar / time picker to set it. You'll also notice that there are error messages if you've not filled in a required form field.
Click on Create and you'll be taken to a screen that says the autoresponder has been successfully created. If you click Go Back, you'll be taken to the Webmail Home again, and you'll need to find and click the Autoresponders item to review.
If it's all done, you should see something like this:
Expired autoresponders will remain on the form, so once you've set up an "out of office", the easiest thing is to edit a prior one.