at first I would take notes on every hand I saw someone showdown
then I figured out I can't cut that by a lot if I just take notes on the important defining features of their game or something that will help me with a tough spot.
If it's something like "called std PF raise from BU w/ 74o, hit MP and called 3 barrels" I prefer just putting "soso bad". Simplifying in that manner helped me a lot, esp cause I ended up getting way more notes on the bad players b/c they showed down more hands.