For creating internal links, start by identifying what are your orphan pages. Now an orphan page is a page that is not linked from any other page.
It is possible that a good theme will always have navigational arrows that link the previous and next articles. So navigation links might make sure that there are no orphan pages but the trick is to also identify "contextual orphan pages". Is there any article that does not receive any links from the main body of other article.
Once you identify these orphans, then you go find similar articles and link from there.
If you don't have similar articles, then consider writing two or three pages that are relevant to the topic and include partial keywords from each other. This way it would become easy to link contextually using long tail keywords.