When coaching product managers, I help them identify complex requirements and decompose them into atomic requirements. That is, requirements which cannot be decomposed any further. As it’s not unusual for newly minted product managers to blend multiple requirements into a single user story, I’ll prepare a checklist which they can refer to when drafting their requirements.
If you work in an Agile or Scrum environment, you might find the ‘flexible’ nature of the sprint/epic/releases can undermine your project estimates. If that’s the case, consider using the following guidelines to identify atomic requirements, which you can then convert into easier to estimate user stories.