Custom email

Hi there

I would like to change the ‘Completed Order’ email so that it includes something like:
“call 07768 123456 to arrange your flight”

I’ve copied customer-completed-order.php to my child theme but am not quite sure (being unable to read php) where to include my custom text.

I read somewhere that a more robust solution would be to write these overrides in my functions.php (again I have no idea how).

Your help and advice would be appreciated.