Step 6: Final layers of concrete
The next layer should be smooth concrete. After a fair bit of research, I came up with this recipe: 1 part portland cement to 1 part silica flour (by weight). Mix them thoroughly dry (don't forget to wear a mask!), and then add water as before, just so that the cement is workable.
Again apply a layer of this cement. You can sand it down to get a very smooth finish, as you can see in the photos.
You'll need to cover this/keep it moist as well, so that it properly cures without cracking.