WE describe the exact steps needed to reduce the problem of planar electrical double layers in aqueous solvent to two simpler problems: (i) a reference system consisting of solvent between the surfaces and (ii) ions interacting with other ions and with surfaces via the potentials of mean force evaluated in the reference system. The mapping is achieved by extending the McMillan-Mayer transformation to the nonuniform fluid between the surfaces. The results indicate approximate additivity of short- and long-range effects in ion density and pressure, as observed earlier in numerical work. After two approximation steps, we arrive at a practical and accurate method for calculating properties of planar aqueous double layers.