Gerotor machines are internal gear hydraulic devices that present numerous performance benefits with respect to other topologies. For this reason, they find their application in high-efficiency mechatronic devices, such as electro-hydrostatic actuators. When designing gerotor machines either as a motor or pump, there are numerous parameters to choose, with different influences on the mechanical and volumetric efficiency behavior. Some of these aspects have been addressed in previous research works, while other important features like the tooth aspect ratio have been partially or completely neglected. Hence, it is the goal of this paper to present a methodology, as linear and generic as possible, to tackle the design of gerotor units. The proposed design method is numerically validated through a case study, where dedicated computational fluid dynamics models are used to assess the performance of the prototype.