I always kind of knew that difference in line weights was critical, so for example the outer contour of the foreground objects in the picture should always be thicker than the inner lines, or the background. It's been pretty straightforward, but somehow I've always fell short of making it happen, since I used nibs with little or no variation in size. I tried the brush, and sometimes it worked, but more often it proved elusive. So, I decided to approach the problem with multiple differently-sized nibs. Of course, I'll still be using brush extensively, but for different contours I'll go with switching between nibs.
And this is how HE does HIS thing: