I actually used notes exclusively while playing. I could not get comfortable playing 16 tables+ with a HUD, so I relied purely on notes.
I used a relatively straightforward system, but was extrememly helpful as the tables were popping up.
First was color coding. I color coded purely on identifying winning vs losing players. I colored winning players from red to light orange (Red=best of the best, Dark Orange=solid winning player, Light Orange=winning, but not great).
I would then color all losing players as Green. Finally I would color wild cards (neither winning or losing, ie. a jagged graph) Purple.
I identified winning players only from stats found on sites like Sharkscope, OPR, etc... That is certainly the easiest way. Losing players can be identified this way as well, however, usually during the course of a game, a losing player will make a play that will display their leaks, so I could mark them Green right away.
As far as the notes, I wanted to identify exactly what they were doing in key spots that would allow me to exploit them. I did not note a lot of Reg's plays unless they did something out of the norm, since I pretty much know how they would play at the various stages of a SNG. However, losing players come in all shapes and sizes, so it will key to identify their tendencies. For example, some guys will limp and fold every time they are raised. Therefore we can raise them frequently to pick up dead chips. On the other hand, there are guys who will limp and stack off with hands like KJ, 22, etc...you need to be careful shoving lightly against these guys. So a note in this situation would be extremely helpful.
Other useful notes (examples):
. Is a guy limping with premiums, AA and KK, to trap
. Did someone just shove 20BB with 44
. Does someone have the check/fold box checked (you can tell if you raise and they insta fold). Could be a great way to steal chips easily
If you play enough games, all of your tables will be full of Orange, Green, and Purple colors, so it will be relatively easy to know just who you are playing against.
Hope that is helpful.
Clint