The ADDIE model has long been established as the primary model used by instructional designers. While on the surface it appears to be a linear process it truely a circular model that puts a process in place for continuing development of a course. Once the course has been implemented and evaluated the designer then goes back to the start, analyzing the results and refining the course.
This video discusses the ADDIE model. The ADDIE model is widely used and often considered the bedrock of the instructional design process.