In this case you are required to create a new correspondence filling in all details as usual.
The only extra consideration you have to bear in mind is to take note of the document number of the correspondence you are replying to and paste it in the Reply To section as shown below.
You can also use the Response Log tool by clicking that tab in the Correspondence Log:
Filters
You can use these filters to have a clear visibility of the documents you have not responded yet or to see which you don't need to send a response.
In this column you will see a '-' if the response is not required or if it has been replied to already.
If you see a question mark '?' a response hasn't been sent yet, so you can either send a reply or click in the question mark to set this document as 'No response required'
In this case you are not required to send a response and as such you have to set this document as 'No response required' as previously explained above these lines.