The post-mortem you will actually write
Post-mortems fail in two ways, and both are about timing rather than insight. Either nobody writes one, or somebody writes a document so long that it is filed and never read, which is the same outcome with more effort.
The version that survives is four lines, written the week the thing ends, while you still remember why you made the decisions.
Four lines
What we expected. Numbers, from before. Six weeks, 18k, tiler in week two. This line is only honest if it came from a plan you actually wrote; if you have to reconstruct it now, that is the first finding.
What happened. Numbers, after. Nine weeks, 21k.
Why the difference. One sentence, and it has to be about a decision, not about the weather. "We booked trades in the order we met them, not in the order the work needs them" is a cause you control. "The tiler was unreliable" is not; it is a description of your luck.
One rule for next time, as a checkbox, phrased so a person could follow it without you. That is the only line that changes anything, and it is the reason to write the other three.
Why one rule and not five
Five rules is a document. One rule is a habit. If a project genuinely produced five lessons, it produced one lesson and four observations, and the four will not survive contact with the next project anyway.
The discipline of picking one forces the ranking that makes the exercise worth doing.
Where it goes
Under the project, not in a folder of post-mortems. The project is where you will be standing when you look for it, and a post-mortem detached from its project is a document about nothing.
Then, when the rule is real, mirror that ONE line into wherever you plan the next thing: the checklist for starting a project, or the Areas branch it belongs to. Mirrored rather than copied, so improving the wording once improves it everywhere it appears.
The pattern after a few of these
Read the "why the difference" lines from your last five projects in a row. That is the highest-value ten minutes in this whole method: most people find the same cause written three different ways, which is a much bigger finding than any single post-mortem produced.
An agent can do exactly that reading and say the sentence you would rather not: "three of your last five say you started before the dependencies were known."
Take the four lines
Open the template, press "Show in my Pando", and fill it in for the last thing you finished. It takes about six minutes, and the fourth line is the one to sweat.