In this tutorial I am going to be making a light icon texture. For this you'll need a camera.
1. Go outside at night time and take a pitcure of street lights, car lights or any other lights. When you take the picture you should move your camera around. The picture I'm going to use is this one here. 2. Then I cropped it to 100x100 but you can skip this step if you don't want an icon texture. 3. Duplicate the base of the picture and set it to something very bright, I normally chose hard light and that's what I did for this one. But you can also do two layers of hard light or screen then hard light, experiment with different combinations of screen, color dodge, hard light, overlay and other settings like those. 4. Now that you've found the right brightness you can right click one of the layers and click on flatten image. And you're done! Below is my icon texture.