The main things to keep in mind when drawing fabric or clothing are resting points, pinches, twists, and most important, gravity. Essentially you want to answer the questions, “where is this coming from?”, and “where is it going to?”

There are a few basic kinds of folds, and it’s worth studying how they look and why they look that way, so that you can break down what you’re seeing when you use reference, and so you can make some predictions of how things might look if you’re working without reference. 

Actually, there is some rhyme or reason to the inert fold, it’s just not really consistent. The fabric is still influenced by all the same things, but it is often reacting to itself. It’s still starting somewhere, and going somewhere else: you just kind of have to take it on a case-by-case basis. I’d strongly recommend using references whenever you can for this one. It’s a lot harder to make up than the others are.

Here are how some of these folds look in action!

Other than the folds themselves, there are few other tricks to keep in mind when drawing fabric and clothing:

Vary your lines

It’s normal to lean toward drawing the same types of folds exactly the same way every time, but it doesn't come out looking very realistic or appealing. Mix it up as you draw!

Avoid tangents

This is a good thing to keep in mind in general, but it’s especially hard when it comes to fabric. Tangents are the nasty little eye-sore that happen when three or more lines meet at a point, or when shapes meet or overlap in a way that makes them difficult to distinguish. They work like arrows, drawing the eye to them, distracting the viewer, complicating the readability of the image, and often flattening the form. Pretty much, they’re annoying, and you don’t want them. Luckily, they can be fixed by simply offsetting lines! … Okay, not simply. It’s a lot harder that it sounds, and avoiding tangents can be a constant battle, but it’s a battle worth fighting.

Of course, the best way to learn to draw fabric is to look for all of this stuff in real life and do your best to draw what you see!