I started with some scrap wood I glued and screwed into a frame and then stapled some old dropcloth. I won't be using that again to paint on, it really does soak up the paint. I had to paint a lot of layers even after the gesso. I put down a night sky-y base and let it dry outside.
After it dried completely thanks to some warm weather, I took a 1" brush and ruined it completely by forcing the bristles to stipple all out. There's more layers of light and dark colors to give an impression of depth.
I took a much smaller brush and dotted the whole thing with stars. Then I grabbed one smaller and kept adding.
I was thinking of the milky way which is just a huge blobby line of stars and colors, so more stippling with white. I let it dry and then poked holes all over the painting and pushed through leds from a string light. During the day it looks like a chunk of the sky.
At night it's large dots of light. I still might add some fiber optics to it but for now I'm happy. I just have to figure out where to hang it.