☞ Answer

SEO plugins usually show a warning about lack of internal links - but it's just a warning. An internal link is a link to another content on the same site. Could be another page or blog post or image etc. No app or plugin is needed to add one.

First, identify relevant content that you want to link to. Let's say blog post A which is already published.

I am now writing blog post B and I want to link to blog post A. To add this internal link:

• Open the blog post you want to edit (blog post B)
• Highlight some text that will become the hyperlink to blog post A 
• Click the link icon in the text editor toolbar 
• Either search for blog post A or copy/paste the link to the post 
• Press Enter 

That's it. You've now linked to post A from within B. 

You can use a button instead of a text hyperlink if that works better for what you're doing.

Want the video?

This was part of a live Q&A and my response starts around 1:45.