If there won't be a lot of text in it, I am thinking PowerPoint or Visio would be a much better application for this task.
If you do not have PowerPoint, then you can download free MS Office alternative called
Libre Office which has two applications called Impress (PowerPoint alike) and Draw that could easily do the job.
Then you could save it as .pps/.ppt type so anyone using MS Office can open and view it too.
A free and awesome MS Visio alternative is
Dia which you can use to make awesome diagrams.
It is hard to open the full installer download site, so it might be better to use the
Portable Dia which I typically prefer anyhow.