Being one of those artists (though my work is not among the best, but still decent), I will say that it takes plenty of patience. For me, perhaps 2 to 3 hours for a regular drawing. I typically look at a manga drawing guide as I draw my head, but one thing to note is that these pieces of art are made up of small pixels, so most artists likely try to draw lines, then add/erase, pixel by pixel, to the lines to make them precise.
If you want to create the illusion of gray, by the way, you need to lay out every pixel checkerboard style in the area you want gray, which means you really need to be loyal to the process, especially if you draw from the smaller 3DS screen. I've heard artists use toothpicks over the styluses (which I use) to be more precise.
That's my advice for Miiverse drawing. ;) And I can deconfirm the magic theory. :P
