This is done easily through your Control Panel. Simply log into your Control Panel, then select "Mail" from the icons, then click on the "Autoresponder" link. Now enter which alias you wish to have an autoresponder created for. Add in the "From" details, typically your name. Leave the subject line as is to have it use the one from the email it's responding to and edit the message body field.
Click on "Create/Modify". That's it.