If your foreground has trees or other objects where you can see stars through gaps in the objects it will take extra effort to produce a mask of the sky. First, remember that the goal is to have all the stars contained in the sky mask and have none of the ground in the sky mask. If some of the sky has no visible stars or there are regions of the image that are too dark to see anything they can either be in the mask or not—it does not matter so this is where you can be sloppy and save some time.
To begin, make sure there are no red dots that are over the trees or other objects. You may want to aggressively remove red dots with a large eraser. Then start painting red dots in the sky around the trees and in any large gaps in the trees. Then zoom in and place dots in any small gaps that have visible stars. When you are satisfied, select “Find Sky”.
If the mask that is generated is close to what you want, you can carry on and edit it. If there are big problems with the mask—for example, the mask covers large areas of the trees—you can select “Adjust Stars” and try to erase or add red dots to improve the automatically generated mask.
Eventually you will likely have to do some manual adjustments to the mask. The key trick to remember is that you only have to mask the stars, not the whole sky. If you see a star in a gap between branches, paint the track it is following—do not try to paint the whole gap. If there are lots of small gaps with visible stars it can be a very time consuming to paint all the gaps. Painting the tracks the stars are following can be much faster.