First, you’ll need an image of some sort of light, with a very black background (see package above). It’ll only work with BLACK background.
1: Open your image in Photoshop.
2: Add the Light image as a new layer.
3: Set the Blend Mode to Screen on this layer (then the black will disappear from the layer and only the lights-colors will show).
The darker an area is on your image, the more light will show.
See a quick video below to see what I mean 😉