Spacecraft in the near-Earth environment typically measure the electron flux at a single position and time. It is the ability to interpret these measurements - in parallel with the variations of the geomagnetic field - that is essential to advance the understanding of radiation belt dynamics. Phase space density is a calculated quantity that can account for these magnetic fluctuations - as it utilises a coordinate system relating to the trajectories of radiation belt particles. This article provides a step-by-step review of the techniques used to convert measured electron fluxes to phase space density, independent from satellite instrumentation and detailed analysis. In addition, examples of how specific source and loss processes might be observed in the phase space density signature are discussed.