Additive manufacturing is revolutionizing medicine with the possibility of creating patient-specific models such as implants, prostheses, orthoses, etc. Customized implants for cranioplasty aim to restore brain protection and improve aesthetics after trauma or malformations. Traditional technologies face several difficulties when trying to produce cranial implants that are lightweight, customized, and resemble bone in terms of weight and structure. Advances in computer-aided design (CAD) and additive manufacturing (AM) technologies, led to a large progress in cranial reconstructions, offering a better anatomical fit, shorter operative times, and greater postoperative comfort, overcoming the limitations of standard prostheses. This work poses a methodology for the design and manufacture of a low-cost AM cranial prosthesis prototype, using specific techniques and technologies such as medical image processing, open-source design software and additive manufacturing technologies. It is thus demonstrated and exemplified the possibility of manufacturing a prototype for a personalized prosthesis at a low cost. The rapid creation of prototypes from medical images and their production through additive manufacturing allows obtaining individually adjusted prostheses before the surgical intervention, reducing intraoperative problems.