All you need to do is a create a rule.
When setting up, select “receiving SMS”.
You can select to forward only SMS coming to Shared Numbers or a particular dedicated number of yours. If you want to send automatic SMS replies regardless of the number they are being sent to, pick “Any Number”.
You can then enter as many keywords as you please if you only want to reply to particular SMS depending on the keyword(s) they contain. If you would like all incoming SMS to be replied to regardless of keywords, leave these boxes blank.
Go to the next step, and select “Send automatic reply”. Your list of templates will appear there.
If you don't have any templates, you can create a new one.
Click Save Rule and… Booya! All inbound messages that meet these conditions will be sent an automatic reply!