Drone operations such as power line inspection, automated deliveries, or crowd control are becoming more widespread. For flights that present serious risks to human safety, operators must conduct safety assessments and get authorizations from the regulators. These preparation tasks are complex and time-consuming but few previous works addressed them. We interviewed 14 professional drone operators, safety study consultants, and 2 regulators to better understand the needs for these tasks. The result is a workable model of the tasks which includes defining the concept of operation, assessing operational risks, and negotiating for authorization. We devised 9 recommendations to inform the design of future mission preparation tools, and consolidated them with a follow-up questionnaire. The recommendations include systematically describing a mission with operational parameters, showing their estimated impact on mission safety, or enabling awareness of the application's status among all stakeholders. We conclude with design concerns and opportunities to inform future research.