In order to create a Sankey diagram, we need to draw an arrow that indicates the energy flux. Also, if some energy is converted or lost, we draw small arrows going out of the main arrow. The width of each arrow indicates the magnitude of the energy.
For example, let's draw the diagram for a light bulb, that uses 100 J of electrical power, but loses 25 J from heat energy, then the useful energy remaining (light energy) is only 75 J: