Awkward but necessary title meaning, you need to periodically re-diagram your processes.
When you first take on a project and because to document the as-is, working toward a to-be state, you will not doubt in some way draw this out. If it’s simple PowerPoint and Visio for the more complex processes.
But you should not stop there, revising only at the end of your project but revisit during and at every changed to ensure you are catching every step that needs to be taken.
This shows everyone the work involved, which your team will always appreciate.
Count on a 1-4 hour task over a few days as you will need a few people to help you confirm the steps, review, revise, etc.