Very easy once you have the cluster on your desk.
Accuracy is not affected, the servos re-calibrate themselves as soon as you plug in the 2nd connector (you'll see and hear it) so if you accidentally move a needle just put it on zero and let the car do the rest when it's plugged back in.
Couple of strokes with a red Sharpie (e46fanatics guys recommended Sharpie over other markers because of the finish and amount of light it lets through) only on the top of the needle as that is the only part that is white, the rest is clear plastic then job is done,