The forensic assessment of testamentary capacity is a topical issue given the prevalence of dementia illnesses and the increase in the complexity of modern financial and family structures. Although a contemporaneous assessment would be desirable, the majority of situations require a retrospective assessment of the mental state a deceased individual. Neuropsychologists, independently or as part of a board of consultants, are, therefore, frequently requested to give an expert opinion of the competence of cognitively impaired elderly testators. This paper offers a guide for carrying out a neuropsychological autopsy, namely the process of posthumous evaluation, for determining testamentary capacity. Taking into account the recent literature on the assessment of testamentary capacity, we propose a three-phase procedure for carrying out the neuropsychological autopsy in the forensic context. The neuropsychological autopsy of testamentary capacity is based on the meticulous, chronologically structured evaluation of any medical documents, available psychometric data, along with the critical examination of any source of information about the cognitive level of functioning of the deceased person at the time of the will.