Select do not disturb.
At the bottom of the Do Not Disturb options is the Auto-Reply To option.
hit this and set a value other than No One.
Select who to auto-reply to.
Now, back in the Do Not Disturb parameters you should see an option listing your current auto-reply message.
choose the current message.
Tap on the current message to overwrite it with whatever you want your automatic reply to be.
input your chosen message.
As normal if iMessage is available it will be preferred over SMS.
