Go to the document screen by clicking the document icon in the main navigation. You should see a "Create" button in the top right corner. After clicking it, select the "BlogArticle" blueprint that you just created.
The document for a new blog article will now open.
Here, you can now input some sample data. Just enter some content so that you can later verify the data in the API and frontend.
After you have finished entering the content, click the "Publish" button in the top right corner.