Start with what people search for
The search box tells you what customers wanted. Look at the most common searches, and especially the ones that return nothing. Each is a question you haven't answered, in the words customers use.
Write for the searches that fail
Turn the top failed searches into articles, titled the way people phrase them. If the words are different from your product's words, add them to the article too, so both find it.
Watch views next to tickets
Compare article views to tickets on the same topic. An article with many views and many tickets doesn't answer the question. An article with few views and many tickets isn't being found. Both are fixable, in different ways.
- Many views, many tickets: rewrite the article, lead with the answer, add steps or a picture.
- Few views, many tickets: link to it from the contact form, the chat and replies, and check the title.
- Many views, few tickets: the article works, so leave it alone.
Ask people if it helped
A simple yes or no at the bottom of each article gives you a signal per article. Read the comments on the no votes first; they often tell you exactly what is missing.
Be careful with deflection numbers
Deflection is the share of visitors who read an article and don't open a ticket. It is useful as a trend but easy to fool, because someone who leaves frustrated also doesn't write in. Combine it with the yes-or-no votes before you trust it.
Keep it fresh
Review the most-viewed articles every quarter and after every product change. An article that is out of date is worse than none. Keep a version history so you can see what changed and when.