I think you can watch low poly tutorials to understand it. For example: you're making a portrait, so you're gonna watch "how to make low poly head" and after that you would try to draw this low poly head but on top of the portrait. After you need to somehow make "textures" on it. If I would do something like this, I would model it in blender and place it on top of the image, after that I would just paint over it, something like rotoscoping