The Famous Artists school started offering correspondence courses in 1948. Although the dress in the material created for those courses is a bit dated, the instruction itself is every bit as useful today as it was then. Here are a few pages which are very relevant for this week's assignment; costuming and drawing folds and fabric.
Click on the images to bring up larger versions to download. All of this information might seem overwhelming but just remember above all else that one side of the cloth always has tension and the other side will be the slack or relaxed side, or you can think of it as one side being the "squash" side, and the other being the "stretch" side. Remember; if you are doing a character that is more cartoony, then you'll want to keep the folds as simple as possible. Economy of line in animation is critical since a character must be drawn over and over again. Every fold must precisely describe what is beneath it and the effects that body type or action are having on the clothing. More illustrative characters, like those in some children's books, graphic novels or game design, can have more detailed folds as a part of their design.
Remember to print these out and keep them as reference for future projects!