The design of a re-usable instructional algorithm for computer based instruction (CBI) is described. The prototype is implemented on IBM PC compatibles running the Windows(TM) graphical environment, using the prototyping tool ToolBook(TM). The algorithm is designed to reduce development and life cycle costs for CBI by providing an authoring environment suited for subject matter experts who do not have instructional skills, and by supporting rapid prototyping. The strategy and tactics are predefined; the instructional developer need only describe the desired performance and the environment of the performance. The specific algorithm described implements a simulation-based reactive environment for learning and practicing device operation skills. Examples would include the operation of many electronic or mechanical devices. The reaction approximates the effects the learner's action would cause in the real environment. The realistic reaction may be augmented by explicit instructional guidance and feedback that would not occur outside the instructional setting. The algorithm begins with a media presentation, then continues with a short tutorial designed to both introduce the procedure and to acquaint the student with some of the learner control capabilities. After the tutorial, the heart of the instruction commences, organized around a set of performances: a demonstration, three levels of practice, and a self-check. Analysis and resource preparation are discussed as two authoring activities. The approach to designing and implementing the algorithm is general, and should apply to other instructional outcomes. (Contains 13 references.) (Author/MAS)