I bet that when you start drawing in Illustrator you only have a few default brushes that are not really of much use unless you customize them to your drawing style. I will point out the basics in using of brushes to create cool outlines in a short time and also point out different line styles that affect your final outcome. So let's get started!
First, free-hand drawing. For this, you need pressure-sensitive tablet and specific brushes that modify their thickness along with the pressure you exert on your pen. We will work on this sketch of a bull.
Open the Brushes window ( Window > Brushes). Double-click a thicker brush and set Diameter to 4 px on Pressure, with variation of 2 px. Leave the rest to Fixed and Roundness to 100%. We use only a small variation to obtain small imperfection that will give the feeling it was hand-drawn. Hit OK. Now double-click another brush and set it to 1.5 px Diameter and set it to 1.5 px Pressure variation. The secret is to use thicker lines on the contour and thinner lines for the details inside it.
In other words, the variation must be half the diameter for the contour brush and 100% variation for thin lines, because we don't want any blunt ends. Here are differences of how a single brush affects the entire outcome. 1 is made with the 4 px brush, 2 is made with 4 px for contour and 2 px for inner details, 3 is made with 4 px contour and 1.5 px inner details. See how much it's affected by just a tiny setting ?
Also if you want your drawings to look good, make sure you have a nice line flow, so your attention doesn't get distracted by blunt areas, un-natural curves and messy areas. When drawing, the rules of aesthetics of a casual viewer do not apply, the graphic language is different and does not count on photorealism. See below the lines i followed to create this character.
I traced the image with the Pen Tool using the same contour brush as above. Notice how blunt and boring it is ? Although it has perfect lines, it does not please the eye with that variation in thickness and produces confusion, and there is a lack of artistic, free-hand touch.
