First, I start with a black background. Then I use a foam makeup sponge and dab it in a very, very small amount of acrylic paint (whatever colors I want for the galaxy-usually pinks, blues, and purples). The sponge is key because it soaks up most of the paint and leaves you with enough to make it look wispy and transparent. Then, for the stars, I use an old, stiff paintbrush (or a toothbrush), dipped in a little bit of water and some white paint, then I brush it with my thumb to flick the paint!
Hope this helps! I’ll try to make a video soon :)