It is as easy than for a regular page. If you are in the main page of the blog (displaying the list of posts), simply click on the "permalink" or "comment" link at the bottom of the post you want to comment.
Then go to the "Comment" section at the bottom of the post page, click on the "Add a comment" button, and start writing your comment.

Then click on "Add comment" to validate it.
Be careful, you will not be able to modify it later.
If you have added a picture in your profile, it will be displayed.
As the author of a post, you are able to delete any comments related to this post.
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