whatever tutorials you've been following are criminal.
how could they tell you to do such things that would destroy the beauty of the original drawings?
image>adjust>desaturate
image>adjust>levels
input levels :- 120 1.00 180
output levels :- 20 255
accept
that is the process for the image you presented.
that will give this outcome.
to adjust for different source images, you need to take the middle slider all the way down next to the dark point, then move the dark point to get rid of the worst 'noise'. then do the same for the light point. then set the midpoint back to 1.00