A general method for the determination of the uncertainty of a physical quantity measured during an experiment is presented. This procedure is described step-by-step so it can be applied to any measurement and experiment available for undergraduate and graduate students, from the simplest to the most complex. The method is then applied to a classic measurement of plasma physics: the plasma potential determination using an emissive probe. This kind of experiment is routinely performed in plasma physics laboratories and may be realized by graduate students. The emissive probe diagnostic relies on a data analysis method called the inflection point. This method follows an indirect procedure on which the step-by-step uncertainty calculation strategy presented in the first part of the article is presented.