I found this tutorial (by RuokDbz98) helpful. It was a time when I was really concerned about making a good lineart (because I didn't and don't have a graphics tablet.) Also, there's the option of drawing the strokes, scan them and tweak them, like in this tutorial, by acsoundwave.
I've found that a combination of brushes (with an eraser to shape it), a radial gradient (set to transparency mode), and median can give me a decent result. One has to brush in a separate layer, and erase it (whether the person wants to create a lineart or paint shapes or shadows.) Then, select the gradient tool, and, starting close or in the middle of the stroke, expand the gradient. The edges should look faded, like this (from XxiMercurYxX.) Then, duplicate the brush layer and merge. And, finally, use median.
Example:
Brush hardness:
0%
75% (default when opening PDN)
50%
25%
100%
No antialiased (or MS Paint-like brush)
Median (radius set at 15 in all of them, only the percentage changes):
30
45
40
35
50 (a higher setting will turn the stroke into an MS Paint-like one)
50 (a higher setting will give it a white outline)
Note: Settings may vary depending on the brush size, especially the settings for median.