If you are using Paint Shop Pro 7 for lineart, there are 3 tools you use: Draw, Flood Fill, and Paintbrush.
For these, make sure your tool settings exactly match what's below:
(if you do not know how to see/access these, see the "Basics" tutorial)
If you are not using PSP7 for lineart, here's what to keep in mind when using another program:
Also, MAKE SURE YOU ARE IN A NEW RASTER LAYER. Make a new layer by clicking the Layer tab, then "New Raster Layer", then OK (See the Layer tutorial for more info).
Make sure you have EVERYTHING ABOVE before continuing. Here, I am creating what Paint Shop Pro 7 calls a "Bezier curve". My lineart is comprised of many of these curves that together make a continuous trace of the sketch it is laid over top of. To make a bezier curve:
Practice makes perfect. Play around with the tool a bit and it'll become second nature to you. The sketched lineart does not have to be completely followed. If you feel a line would look better another way, it's okay to deviate from the base sketch (I do this all the time).
Keep in mind, with my style, the curved lines are meant to outline only the VERY EDGE of the sketched lines. The space inbetween is filled with the Flood Fill tool .
The example to the left demonstrates all of these things said above. I also added flat coloring to this example to show what will eventually be done with the lineart.
NOTE: If your flood fill does not effectively fill inbetween the lines, you may have not made a new raster layer. If your fill spills over the entire image, you likely missed a tiny gap in the lineart that would allow it to "leak out", which can be common. The lineart must make a completely enclosed area.
Not all of my lineart occupies the same layer, so make sure you are always in the correct layer when doing lineart (see more in the "Layers" tutorial). Why? Because I add layers based on position relative to the viewer, some pieces of lineart will be on different layers of the image (example: hair going in front of a character's eyes would be on a layer above the base character layer, but hair going behind the head would be on a layer behind the base character layer). Why do this? Because it is easier to make adjustments to the lineart/flat coloring when its layered than it is when all of the lineart is in one layer.
The animation to the left shows how I am layering my lineart for this example image. It's a pretty strange way of doing lineart, but I've been doing it this way for years. It has its limitations, namely with the lack of anti-alias, but my images are generally so large in resolution that this issue resolves itself once it is resized for the Internet.