Use three stages
Before leaving
Permissions, weather trend, paperwork, batteries, spares, maps, client brief and obvious equipment faults.
At the site
Actual conditions, people, hazards, airspace/location changes, launch/recovery area and crew communication.
Immediately before start
Final configuration, control/navigation status, failsafes, mission objective and stop criteria.
After the mission
Damage, battery/equipment condition, defects, unusual events and anything that should alter the next checklist.
Add stop points
A good checklist includes reasons not to continue. Examples are an unexplained equipment warning, missing authorisation, weather beyond limits, no safe recovery area or a crew member who is not ready.
Write down the contingency
For each mission, identify the most likely failure or interruption and decide the response before it happens. That turns a checklist into a decision tool rather than a memory exercise.