Please navigate to the tickets tab and open the ticket that you would like to work on. Kindly click on reply button within the "tickets details page" which would allow you to type out your contents. Among the various formatting options, you would find a "book" symbol that would lead you to the solution articles. You could either search the relevant article or choose among the ones listed here.
Please make sure the article is visible in the customer portal if you are sharing a link to the article. Internal articles can also be inserted into the responses by choosing the "content" option within the article suggestion.