Most executive summaries are written last, as an afterthought, which is exactly why most of them fail, they end up as a compressed table of contents instead of an actual summary of what matters.
A table of contents tells you what's in the document. An executive summary tells you what to think and do about it. That distinction is the whole job. If someone reads only your summary and nothing else, they should walk away knowing the situation, your recommendation, and what you need from them, not just a list of the sections that follow.
"This document covers our Q3 performance, an analysis of the market, three potential strategies, and a recommended path forward, along with implementation considerations and risks."
"Q3 revenue came in 8% below plan, driven mainly by a slower-than-expected enterprise sales cycle. We recommend shifting two headcount from mid-market to enterprise support in Q4 to address this directly. Full analysis and alternatives considered are below. We need a decision on the headcount shift by October 1st to hit the Q4 timeline."
The second version could stand entirely on its own, a reader who stops there knows what happened, what you're proposing, and what's needed from them. The first version tells them nothing they couldn't have guessed from a table of contents, and it forces them to read the whole document just to find out what you actually think.
If someone only reads your first paragraph, would they know what happened, what you recommend, and what you need from them? If not, it's not a summary yet.
A useful test while drafting: write the summary as if the rest of the document might get deleted. Would the reader still know what to do? If the summary depends on the reader continuing into the body to actually understand your position, it's still doing the job of an introduction, not a summary.
Length discipline matters here more than almost anywhere else you write. A summary that runs half a page is usually still trying to explain rather than conclude. Four to six sentences is enough for most business documents, the situation, the recommendation, the reasoning in brief, and the ask. Everything else belongs in the body, available to whoever wants to go deeper, not required reading for whoever just needs the headline.
Write it last, after you've drafted the full document, you genuinely can't summarize conclusions you haven't reached yet, but treat it as the most important paragraph you'll write, because for a meaningful share of your readers, it's the only paragraph they'll actually read.