The optimal planning of electromagnetic skins (EMSs) installed on the building facades to enhance the received signal strength, and thus the wireless coverage and/or the Quality-of-Service (QoS) in large-scale urban areas, is addressed. More specifically, a novel instance of the System-by-Design (SbD) paradigm is proposed toward the implementation of a smart electromagnetic environment (SEME) where low-cost passive static reflective skins are deployed to enhance the level of the power received within selected Regions of Interest (RoIs). Thanks to the ad hoc customization of the SbD functional blocks, which includes the exploitation of a digital twin (DT) for the accurate yet fast assessment of the wireless coverage condition, effective solutions are yielded. Numerical results, dealing with real-world test beds, are shown to assess the capabilities, the potentialities, and the current limitations of the proposed EMSs' planning strategy.