While industrial competition is intensifying and thus encourages engineers to directly match projected systems with user requirements, time to market is now also shorter. Designers are therefore endeavoring to validate their system models as early as feasible in the development cycle -- where possible, right from specifications. Prototyping offers practical answers to these two constraints. Additionally, Component Based System Engineering (in short CBSE) promotes rapid development of systems, with emphasis on reuse. This paper presents a UML-based methodology combining both approaches. The prototyped application consists of a main component or "application component" and a set of external components representing the environment. After explaining how to prototype a component, we describe connection patterns that facilitate integration of heterogeneous components within the context of ACCORD/UML. Throughout the discussion, ideas are illustrated by a simple case study based on an automotive navigation system.