I'm not confident about my english guys, sorry if I did mistakes~ This is just an example of a night sky. you can find a better, greater night skies in photographs. so I recommend you to browse a lot of photos for inspiration for example, the shape of the milky way, I did a very funny one in this tutorial but there are many ways to create them. let your imagination runs wild <3 and before you ask, yes, I only use Paint tool SAI. no other programs :'D
good luck, and I hope this tutorial helps~
update 1: you can also copy and paste instead of using dulipcate layer option. if you dont have addition blend mode on your SAI I recommend you to try luminosity I think they're the same.