If you simply remove the lower from the gun, you can manually cock the hammer, pull the trigger and see how things work.
With the hammer cocked back, two hooks on the front side hold the trigger cocked and ready to fire. These hooks are the sear. Polish under them and polish the hammer "wings" they hook onto.
Pull the trigger and "catch" the hammer with your thumb so it does not crash into things. With the trigger still pulled all the way back, cock the hammer again. A hook on the rear catches the hammer on a rear shelf. This hook is the disconnector. The limits the rifle to one shot per trigger pull. When you let go of the trigger, the rear disconnector lets go ("trigger reset") and the front trigger hooks then "catch" the hammer, ready for the next trigger pull. Polish the bottom side of the disconnector hook and the hammer shelf surface it rides on.
There is a long lever to the front right side. Polish the lever on the side against the wall and the wall itself.
Just by looking at the trigger assembly and working it a bit you can see what is moving against what and tell what needs polishing. Ok, maybe that long lever is not so obvious, but I find it to be "the" major source of drag in the assembly when I pull the trigger on a "fresh" trigger assembly. The HBI spring set has a thin washer that moves the lever slightly away from the trigger wall to try to reduce this friction.
CZ seems to coat everything in a black material that is not smooth. Thus the "gritty" feel. In general you are removing the black gunk and exposing shiny metal underneath.
I can see where firing the gun a lot might naturally wear these trigger surfaces smooth. You could likely "dry fire" the lower all by itself 500 to 1000 times. I am not sure if you would smooth out the trigger before you wear out your thumb catching and cocking the hammer, but dry firing could save some ammo.
- DanT Phoenix, AZ