This tutorial gives both general drawing tips and program specific tips to Krita 2.9 and Paint Tool Sai.
Giving your lineart weight differences will make the drawing more alive, especially if you're looking to give the finished image a more simplistic shading or maybe even have it black&white or flat coloured, and it's still look nice! Of course, not all lined drawings need to have weighted lines, such as only thin lines on softshading is a much better combination than weighted lines with softshading!
In this tutorial I'll tell you how to plan out where you should place thicker lines and explain tools that help you draw a smooth lineart!
The right brush
First off you need to have the right kind of brush and size of canvas to start. A sharp edged brush with only little anti-aliasing that reacts sensitively to pressure, ranging from just 1-2px into 25px. Assuming we are working on a 2000px to 4000px canvas size (if your canvas is bigger, your brush will need to be bigger too).
The brush I use in Krita is Inking Dyamic Pen (new) from Woltheras inking brush pack, you can find the pack from here: Woltheras brush thread on KDE forums. You can also mod your own brush, but it's faster to download and you get a nice icon for it too.
If you're using Sai, you can mod your own suitable brush from the default pen. Here's settings that I used back in the days, also used by SqueedgeMonster who I copycatted the settings from.
Your best friend: Stabilizer
Linearting doesn't have to be horrifying repetative progress of trying to continuously draw a nice smooth line and spamming ctrl+z until that happens. Great programs ment for digital drawing have two important tools that improve the progress a ton!
First one is the stabilizer, if you have a twitchy hand or just feeling lazy. Stabilizer smoothens the line drawn, but it becomes laggy if you're using it on very high settings! Sai's stabilizer (sometimes called smoothing) is on the toolbar above your canvas, it has settings from 0(none) up to 15 (very strong) and S modes that are even more radical but also very laggy. I suggest using something between 4-8, since it'll still feel natural to draw with that.
Krita 2.9 has a bit more features and settings to it's stabilizer. You can find it when you have the drawing tool chosen, under 'tool options'. If you want the same simple stabilizing as Sai: choose 'stabilizer', tick off delay and keep the distance around 50. Delay option will give you a circle around your brush area, it will only draw until you've dragged your pen off the edges, this is very useful for long super smooth lines! Assistant will snap your brush to rulers & perspective grids, I'll explain that part last.
The other stabilizers in krita are 'basic smoothing', great if you're drawing very fast but your computer can't quite keep in track with your pen movements, it smooths the line and prevents jitters and glitches but doesn't lag. Weighted smoothing has options to smooth lineweight, if you're bad at controlling the pressure choose this and tick the 'smooth pressure' on and give some numbers to 'stroke ending' and it'll prevent odd blobs in your line!
The assistant! Before you tick it on and go you'll need to create your ruler/curve/grid. Choose the triangle-like ruler tool instead of drawing tool. Then look at tool options, a drop down menu should have several different options, they're all pretty useful. Try out the one called "Spline" if you have trouble drawing curves free hand but find vectors as slow and terrible as I do! Once you've created your stuff, go back to brush and tick assistant on from the smoothing tool of your choise. You can delete your rulers when you go back to the tool.
The correct kind of eraser
The second important tool is being able to switch to transparent colour to erase with the same brush as you're drawing with. If you move to a different brush for erasing, it may have different softness and anti-aliasing as your inking brush and that will look reaaaaally ugly!
Find the eraser switch from the top toolbar in Krita, or where ever you have decided to place it. In Sai it's the tiny square under the selected colours. Use the eraser to correct your lines and make thinner parts thinner. Not even professional artists can do super presice lineweight with one stroke, instead they usually give in for small mistakes (that's a good method too, but now we are looking for the amazing features digitally arting can do!)
Where to place thicker lines
Firstly, your lineweight should be defining where darkest shadows are (thick line) and parts directly hit by light (thin line). Before starting, mark down your lightsource or keep it in mind (once you practise more it's usually easy to keep in mind). Here's a picture showing what parts in this piece should be thicker.
Next comes in the base colours of your characters & objects. Give thicker lining around black or very dark materials - such as black fur! This also helps seeing the lines on spots that will be coloured black, if you do linearted work your lines shouldn't blend into the rest of the piece, otherwise it's a bit pointless putting a lot of efford into them, right? Especially define parts where two different dark materials touch eachother, so they can be told apart. Here's a picture of which parts in this same piece should have thicker lines because of the base colours:
Third point to coinsider is distance of your characters and objects. Things close should have thicker lines, that will help define depth. Subjects very very far away are worth coinsidering to leave without any lines at all, such as pictures that display background all the way to the horizon. Make your brush's max size smaller when linearting objects further away, so you don't accidentally give them lines that are equally thick as the front stuff. Here's a pic that illustrates which parts of the following pic should have difference between the overall thickness of the line by distance:
When you're done with the basic lineart, it's time to give a little more detail to it that'll truly bring it alive! Of course this can also be done along the progress, but it's better to perhaps practise it first separately.
1. Lines that collide into others: When it's part of the same material (such as fur in this case), I thicken the base of the line hitting the other, this nicely blends the lines together and gives the material a more solid look. However, if it's two lines defining different materials, I wont thicken the lines, or might do it very mildly - to keep them looking separate!
2. Angles: I usually thicken angles, it helps to highlight them out, especially if the angle isn't otherwise so visible. This should be done in good taste, test out which angles look good when thick and which look odd.
3. Lines that end: When you have a line that ends to nothing, try to thin it as much as possible and leave the tip very sharp.
4. Big dark areas: Place larger black areas to where no light reaches, and especially if the base colour of the shadowed area is black or dark! But keep in mind that putting one big area like this or many around the same spot will leave the results feeling unbalanced. Try what looks best!
Overall the variation of line should look nice and balanced, so scan the piece through and modify parts even if they'd go against the basic light & shadow rules - drawing isn't always about being exact to reality, you should always take freedom if it generates a better results. One nice touch is also to give your highlighted character or object thicker lines than the rest of the piece.
Finally, here is the finished lineart of the piece used in the tutorial: