Here is my formula:
First paragraph - provide a concise summary in my own words what the authors sought to accomplish in their manuscript, the strategy they took to accomplish that, and what the outcome was. I include no evaluation in any fashion. Rather, the point is so provide an executive summary for an editor.
Second paragraph - cover what makes the paper strong. I keep in mind that, particularly at high impact journals, a paper won’t be accepted unless a reviewer specifically goes to bat for it. At the end of the day, editors want to publish good papers, not just not-bad papers.
Third paragraph - briefly (1-2 sentences) summarize any substantial concerns I have along with an indication of the severity (deeply concerning / concerning / mildly concerning). Importantly, these concerns and the severity are exclusively in regard to how conclusive the findings are or regarding the integrity of the approach. This paragraph ends by saying “I will unpack these concerns one at a time under the subsection ‘major concerns’ and enumerate minor suggestions for further improvements under subsection ‘additional comments.’
Next I have a subsection with one paragraph per major concern. For each I concisely name the concern, explain why this concern is concerning (risk / cost of leaving it un-addressed), and offer constructive input regarding how I could see the concern addressed.
Finally I have a subsection for additional comments. These are usually in bullet form. This concludes things like “a legend in figure X would be useful” or “use of the term X was confusing” etc.