It is actually a fairly simple process, but it depends very much on a good eye for balance and composition.
The first thing to do is get the night sky look right. The effect won’t look right if you just paint stars over black, so you have to go in there with some dark blues and violets to try and form dark clouds.
This image shows the blue/black contrast very well. I started out by painting some dark blue over parts of the black orb, and then putting some blue ink over it all before adding any of the stars.
The next big step is to add in the stars. For the most part, you only want some non-uniform tiny white dots across the sky. If you know astronomy well enough, you can throw in some constellations, but to most of us, it will look right so long as the stars don’t look too balanced or too much in line.
After that, you will need to pick some things to add centers of interest. The star scape is never enough by itself.
The easiest detail to add is a bright star. Don’t worry too much about realism here, what you’re going for is the star that they print on Christmas cards.
You might want to consider a few other options. The Woldwyrd above has a comet on it- something very small and simple, but it gives just enough point of interest to make the starscape work.
The opposite way to go is to use the moon. A simple sphere or crescent shape isn’t quite all you need. The moon in the sky is not quite all white, so the best way to treat it is to use a little off white bone or even yellow layered underneath to provide texture.
After that you paint some white over portions of the moon, giving it a strong shape.
There are a lot of places that a starscape can work, and plenty of times you won’t want to use it. Try it on parts of the mini that are largely blank, and yet need to be centers of interest.
That’s all of the basic techniques. From there, try experimenting with different nebulae or effects, and see what you can come up with.