This paper presents a methodological proposal called UP4VED (Unified Development Process for Virtual Environments), a development methodology which is based on the Unified Process and best practices for building virtual environments; this methodology includes the best proposals among the existing methodologies for the development of Virtual Environments (VEs), special considerations for its modeling, the pillars of the unified process, as well as suggestions from the SPEM 2.0 OMG standard. Furthermore, the development of an application called "Platform for Assisted Generation for Virtual Environments (3DGEN)" which employs the methodological process UP4VED.