Make sure you keep all the photos for your star trail stored in their own folder with the original numbering of the image files so that they are added in to your star trail assembler correctly.
Then choose your software. There are several options out there, from star trail-specific software such as StarStax to a more general photo editor such as Photoshop. For this tutorial let’s look at the Photoshop option.
Import your photos.
Open Photoshop and go to File –> Scripts –> Load Files into Stack.
Select your folder of star trails photos, highlight all of the photos and then select Open followed by OK.
Blend your photos.
Once Photoshop has added all of your photos, select them all in the Layers panel and then go to Blending Mode and select Lighten. Hey presto! You now have a finished star trail photo. If you like you can now go through and manually deselect or adjust the layers.