1) Tripod
You need to have a tripod or something you can have your phone stationary on because you will be exposing your camera for a long period of time. If you handheld the phone, the image would be blurry.
2) App
It is easiest to download an app that lets you manually set camera settings and that allows long exposure. You need to be able to manually set shutter speed and ISO at least. Even better if you can set an f stop manually. If it lets you shoot in raw, now you are talking.
3) ISO
If you are able to set ISO manually, set it to 6400 or above. You will need to bring it up even higher if your image is underexposed.
4) Shutter Speed
Most phones have a focal length of around 25mm equivalent. So you should be able to have shutter speed as long as 20 seconds or so to prevent stars from trailing. If you are shooting star trails, this does not make that much difference.
5) F stop
If you have a capability to change your f stop, change it to as low as it goes. If not, you need change ISO and shutter speed to properly expose the images.
6) Timer
You need a self timer on your phone to avoid motion blur caused by pressing a shutter bottom on the phone screen. Put it on a self timer. 2, 5 or 10 seconds do not matter.