Documentation that is useful to a reviewer is the same documentation that is useful to your team: it shows what a child was learning, how teaching responded, and that someone came back to it. A folder of observations nobody revisited shows effort, not practice.
The simplest preparation is continuous rather than seasonal: notice every child regularly, write next steps you actually return to, and record family voice where you have it. StoryLoop's evidence pack counts those things from stories your team already wrote and names the gaps first, so they can be fixed while they are still small.