The use of Augmented Reality (AR) in training or assisting operators during an assembly task can be considered an innovative and efficient method in terms of time saving, error reduction, and accuracy improvement. Nevertheless, the implementation of an AR-based application is quite difficult, requiring to take into account several factors. This paper provides a general procedure to follow for a correct implementation, starting from an assessment of the assembly task, until the practical implementation. To assess the procedure, it has been applied to the training of unskilled operators during the assembly of a planetary gearbox, with the help of a hand-held device.