Protein liquid-liquid phase separation (LLPS) has been associated with biological functions and pathological aggregation. Mapping the phase separation conditions is the first step to identify and quantify the driving forces of LLPS. Here, we describe the protocols to draw the phase diagram of tau-RNA LLPS and use the mapped diagram to guide experimental conditions for LLPS-cell coculturing, electron resonance spectroscopy in particular double electron-electron resonance spectroscopy, crosslinking immunoprecipitation, and isothermal titration calorimetry.