Title: Analyzing a Research Paper: A Critical Review

A five-paragraph write-up (one paragraph summarizing each of the paper’s four major sections — introduction, methods, results, and discussion — plus a fifth paragraph critiquing the paper as a whole) would work well, but you can decide exactly how you’d like to structure your own write-up (being sure to include all of the components just mentioned).
As long as you can demonstrate that you’ve read the entire paper and can interpret and analyze the objectives, procedures, and conclusions, the exact write-up length is flexible.