The study of the methodologies useful to support the assembly of parts is a challenging engineering task which can benefit of the most recent innovations in computer graphics and visualization technologies. This paper presents a proposal for an innovative methodology based on Virtual and Augmented Reality useful to support the components' assembly. The herein introduced strategy is based upon a four stages procedure: at first the designer conceives the assembly sequence using a CAD system, visualizing the scene wearing an immersive Virtual Reality device. In the second stage, the same sequence is developed by an un-experienced user using the same equipment: the differences between two assembly sequences are recorded and exploited to detect critical points in the assembly sequence and to develop a Knowledge Based System. Finally, a virtual user manual is produced in Augmented Reality. When the final user uses the tool, the position of the object to assemble is detected by tracking the finger position of the user itself. A series of symbols and writings is added to the external scene to help the end-user in the assembly procedure. A test case based on the assembly of a scale model has been developed to evaluate the methodology. After an evaluation process, the procedure seems to be feasible and presents some advantages over the state-of-the-art methodologies proposed by literature.