By definition, content marketing provides a useful (and usually free) resource for people searching on the net. So the first step is to research what topics people are searching for that are relevant to your product/service. I do this with various keyword tools (e.g. moz, ahrefs), and by looking at forum discussions about those topics (Quora is great for this). Then if we have the expertise to offer something useful, we write great content optimised for those topics and searches.