First, open the blog menu in the site editor:
To add a blog to your site, open your site in the editor and open the page picker (A). Then click on "Add Page" (B).
If you already have a blog, you may see the "Blog Home" and "Blog Post" legacy templates. You can delete those after completing this setup.
Then either pick an existing blog if you have one, or create a new blog:
Follow the steps in the page picker to either select and existing blog or to create a new blog. The process for creating a new blog can be completed in the editor as well.
After selecting or creating a blog, it's time to add the blog templates to your site:
Enter the base path for your blog setup. We will then create three new pages for you: the blog homepage, the blog post page and the blog archive page. You can change these urls after creating them page.
The only required page for a blog setup is the "Blog Post" page. The homepage and archive page are optional, although typically used for the classic blog experience.