One of the most important areas in building a simulation is to start small and focused, and then make small, progressive changes. Keep it simple simple (KISS)!
One area that simulation educators continue to have problems with is staying focused on the big picture when building a simulation scenario. Remember to keep things simple and straight forward.
It is critically important to decide what the learning objective is (pick one or two at most), and then stay focused on that objective and build the scenario around it. Educators tend to want to consider every detail and try and plan for everything. However, I'm sure most experienced simulationist experts will agree, it's impossible to plan for everything and this is not something that's important or that really matters. The most important aspect of a simulation experience is to identify the learning objective and stay committed to it.
At the end of the day, the simulation will be debriefed and learning for the participants plus the planners will occur. The reality about simulation is that with each iteration, small changes to the simulation can be made and these changes tend to have a bigger impact on the direction of the planned learning objective.
In sum, one of the most important areas that I have learned with my experience with building a simulation is to start small and focused and then make small changes and note the effect of those changes. Keep it simple simple (KISS)!