A good postmortem should include:

• A brief summary
• A timeline of the event from discovery through investigation to resolution
• The primary cause of the event
• Impact and damage assessment
• A set of action items to fix the problem immediately
• A set of action items to prevent the even from happening again
• Lessons learned

See https://landing.google.com/sre/sre-book/chapters/postmortem-culture/ and https://landing.google.com/sre/sre-book/chapters/postmortem/.

See also: https://github.com/etsy/morgue