To use Startrails, for example, simply download the program and copy the folder to a location on your PC (the program requires no other installation). Run Startrails by double-clicking the icon in the program folder.
Select File > Open, and choose all of the photos from your star shoot. Then, click Build > Startrails. A few minutes later (it might take a while on a slower PC, or if you have hundreds of photos), you'll end up with a completed photo that you can save to your PC.
Note: This software is donationware. It's free to use, but the developer accepts and encourages donations toward further development. For more information on how to make star trail images, see "."