The research on ion-measuring devices has seen a rapid expansion in the last years as a consequence of the growing interest in wearable sensors for biofluids analysis and in portable devices for remote or in-line water and food quality monitoring. As a result, an increasing number of researchers is approaching the field of ion-selective sensors. Despite the apparently simple transduction principle, the theory behind their working mechanism is far from trivial and a correct understanding is necessary for an optimal exploitation of the technology. In current literature, imprecise characterization procedures lead to the definition of misleading sensors parameters, which cannot be effectively used for comparisons. In fact, some unique definitions and procedures applies to this category of sensors, which significantly differ from the traditional ones applied in sensing research. This tutorial aims at highlighting the basic thermodynamic theory and the correct experimental practice for the accurate and reproducible characterization of potentiometric ion-sensors. The most important requirements and design considerations on hardware and software interfaces will also be discussed to give a complete overview of the various aspects of current technology to people approaching this promising area of sensing.