I also had a hard time deciding how to filter/organize/theme my W&A section. I ended up using the AAMC medical student core competencies as a guide (
https://www.staging.aamc.org/initiatives/admissionsinitiative/competencies/). This was my basic plan for the W&A section:
-There were experiences that I knew I wanted in there, and some of them were multi-year experiences. In that case, I used the competencies as a way to narrow 2,000 hours of a broad experience into 700 characters.
-After I had the big ones in there, I identified the core competencies that weren't yet represented in my application, and then used that factor as part of the decision process for deciding what else to put into my W&A section.
-I tried to use concrete examples as much as possible to show why a particular experience demonstrated a particular competency or set of competencies.
-For each entry, I tried to speak to at least one of the following three things: why the experience was impactful, how it helped me grow, how it helped me decide on medicine as a career (the "so what" factor).
-I intentionally left off some "achievement" themed entries in favor of something softer / more personal (hobbies, in my case), which was the exception to the above bullet.
I'd also recommend chipping away at these early, as the whole application is a bear for us non-trads who have a decent amount of life experience to try and articulate. Personally, I used my PS and W&A section as a productive way to rest my brain from MCAT studying.
Hope this helps!