For work, it's worth putting together a template/checklist so you can make sure others are on the same page regarding content and expected clarity of the document (and the process of creating and reviewing it).  For personal look-backs, it can be a lot less formal, but it's still nice to have a list of things you want out of it.

Important (IMO) sections:

• "what happened".  A few paragraphs of plain-language explanation of what you're looking back on.  Should be somewhat narrow and specific to a single event or decision, rather than "last week felt bad, here are all the reasons..."
• "why do we care".  A description of the impact or cost of the event or decision, perhaps with a comparison to what SHOULD have happened, perhaps with a description of risk even if there was no cost.
• Timeline - datestamps of when things happened, including when we reacted or learned about things.
• "how did we behave during the problem or event" - time to discover, time to mitigate, time to resolve, how clearly were we focused, what confusions caused delay or waste.
• "formal root-cause".  Often called "5 whys", for the idea that it takes (at least) 5 levels of causation to understand something, but other formats work as well.  Do go deep enough to understand the systemic or structural reasons for the thing.
• "What should we change for the future?"