Hi iwanmahe. What do you mean, you have no idea how to use it? The Lightbox just searches your html output for links to images and make them to a lightbox. You don’t have to do anything. You can change some options.
If you want to use it anywhere else, just try to use:
https://github.com/andreknieriem/simplelightbox on your own.
I will update the plugin with the option to add Javascript Code on your own and maybe some shortcode, if there is a need for that.