But lets see first disable the use of pressure for opaque and make sure size is enabled.
If you want to control opaque (for painting) then disable size those two dont go that well atleast in my eyes.
Brushsize around 5-10 depending on how thick the lines need to be.
Hardness is a matter of taste 0 for a softline look. I prever stuff around 50 and up for inking tho.

As for the ink fast advise the other gave.
Usually I would agree even if I dont do that myself (yeah bad habit), psp however doesnt interpolate drawn lines, so when you go fast it often makes stright linesegments.
For psp I can recomend zooming in at 150-300% and slowly filling the lines by doing short strokes.

Oh and if it isnt clear by now psp brushengine sucks ass, only gimp is worse as far as I know^^.
What I'm doing lately is using opencanvas 1 (the free one) for inking and sketching for that its brushengine is very good. coloring tho still happens in psp.

And a last tip I picked up not too long ago.

Practice the lines two or three times, hold your pen close to the tablet but dont press on it. that way the cursor moves without drawing a line. this way you can see if you're following your scetch/pencildrawing. if you got the line right in your mind do it once more with fee... eh with pressure .

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Wer nicht vom Meister lernt, lernt nie etwas.