This technique looks quite good once it's done, but is in reality quite simple. I'm using an empire wizard I had lying around because most of the model is cloaked.
It happens to have been sprayed with macragge blue, but you can use a black undercoat to save time.
Remember to do all steps shown to the inside of the cloak as well, unless you plan on having a different scheme on the inside.
First make sure there's a smooth black coat over at least the top half of the cloak, if you've based it all black that works just as well. I decided to make the line of the black slightly diagonal.
The mix about 50/50 abaddon black with naggaroth night and add some water.
Paint a thin coat from the bottom up to overlap a few millimetres into the solid black. As you apply more thin coats to build a solid cover, go a shorter and shorter distance up the cloak to make the dark purple strongest at the base. This helps add a subtle twilight effect.
Take a large drybrush, a small drybrush and select a few different colours to paint the nebula. For each colour, say green, red, etc, you'll want a darker and a lighter shade, here Kabalite green as the darker shade, then moot green for the lighter.
You an also just add white to a colour. I made a 50/50 mix of white scar and naggaroth night for a darker shade then a 75/25 mix for the lighter.
You don't want too much paint on the brush, but you want more than you'd have for drybrushing. I scraped the tips of the bristles into the paint then smushed it up a bit on my palette.
Then you want to brybrush/dab the dark paint on in splotches and lines until you get a fairly strong colour. Do all the darks before moving onto the lights.
Take the small drybrush and put paint onto it in the same way as before, then dab on the lighter paints inside the areas of corresponding darker paints.
Before painting on the stars, make sure the nebula is done both inside and out.
Take an old toothbrush and dip it into some white scar, getting just a bit onto the bristles, not too much. (Hopefully you can see the small drops on the bristles.)
You then want to run your finger across the toothbrush, flicking off specks of paint. practice first on some newspaper or a scrapped model so you know you've got the right amount of paint and are drawing across the bristles with enough force.
I apply 3 to 5 flicks because I like my night sky extra starry, but if you want less stars do less. (
Tbh I may have gone slightly overboard here.)
Now you're gonna get some larger drops that stand out and/or look kinda off. This is good. Take a small or extra small brush and mix some white scar with some water, maybe 75/25, or any amount you prefer to work with. The point is the paint must flow easily off your brush.
Then go to each stand out spot and paint a little cross over it to emphasise the star as one that shines particularly bright, as above.
And voila, one night sky transported onto your cloak or cape of choice.