Blog posts are added automatically if you have provided your Blog URL in boat setup.
When you submit a waypoint, Farkwar looks at the datestamp for that waypoint, and the one before it. If it finds any blog posts that were written during that time, if automatically links the posts to your penultimate waypoint. This is editable, but I think it should be right 99% of the time.
If you would like to manually submit a blog post for a waypoint, you can do so by following the "Add a New Post" link on any existing waypoint.