Dark posts are created using the somewhat clunky, very obtuse, Power Editor. I won’t go into a tutorial on obtaining and using the Power Editor as there are some fine ones here. If you have no idea what the Power Editor is you should read one of these tutorials before going much further.
If you are a Power Editor user you simply need to play around with a few more features.
The key to creating dark posts in the Power Editor lies in the editor’s ability to create posts. Until the dark post concept came along there really wasn’t much reason to use this feature as you could simply create posts as status updates.