How to Add a Blog to an Already Existing WordPress Site

If you built your site yourself and know some CSS it won’t be too difficult for you to fix this issue.
Install the WP site in a folder, like https://example.com/blog so /blog will be in the URL of all your posts.

Then add the blog to the navigation on all your HTML pages, linked to https://example.com/blog.
But that is not the end of the story. To make it complete you have to do a few more steps. Here is an article, which might be of some use for you to understand the working basics.

https://jonathanwold.com/tutorials/wordpress_integration/ Good luck with it!