I've done several of these over the years and they're basically as has been described here, above.
I've found it helpful to start reeeeally simple - just the barest outline/ overview - because like you wrote, otherwise you just get flooded and lost in the details... And then to go over it later, and add bits in that seem particularly salient - sometimes even adding things in with my T as we're discussing it and they'd point something out that I'd forgotten or they'd ask about something and we'd add in whatever I answered...
To me, it's kind of like a "map" of what happened in your life (with the focus on traumatic events/ healing from traumatic events e.g. moving out of a toxic home, breaking up with an abusive partner, etc) and just like if you crammed a map full with "all the details" then it would be "very informative" but basically "too" informative, cos it'd be a flood of information and then you can't "see" or "read" the most important stuff on the map, if that makes sense?
So to me, less is more, at least initially (so not trying to cram in "everything") and then giving your brain time to figure out which information would be informative/ helfpul/ interesting/ important to add and what would just be adding "noise" in terms of an overview...?
No idea if that's of any use or whether it makes it sound more complicated...