Of course the big consideration in this painting is How Do I Paint the Stars?
The versatility of watercolours allows us to use a variety of methods: when the painting is complete then apply your stars by: taking a toothbrush and load the bristles with gouache. Then use your finger to spray the paint off the brush.
Or, take a large brush, mop head, chisel or fan brush, load the end of the bristles with gouache and flick the brush on to the paper, working top to bottom and covering your painting where you do not require stars.
Or, use some rock salt. Paint your sky wet on wet and when the paint is still 50%-75% wet sprinkle the rock salt on to the sky area. Let it dry 100% before you attempt to remove it, otherwise it will defeat the object and you may smudge the paint.
Or, and this is the course of action I take, use some glitter. Glitter comes in many different formats but the one you should use is one that is suitable for household painting, which you purchase in a large DIY store, not the sort of glitter you purchase in a craft shop. There is a vast difference in the staying power of the glitter: the difference in the price of the craft glitter and the more industrial type glitter is an indicator of the quality, the one I buy is not inexpensive but it is worth every penny. I do not use it on an everyday basis so it lasts me a long time.
When I painted in the sky I worked in sections and when the paint was wet I applied my glitter with a small round headed brush.