The Best Fashion Illustrator Book
Sure, you can take expensive Adobe Illustrator classes until you are out of money and blue in the face. No class is going to teach you what you need to know to create fashion flats and artwork repeats, unless it is an Illustrator class specifically for the use in the apparel industry. The best way to learn the program and keep up with industry standards and to get a really good book and take apart existing flats and prints that have already been created before in the correct format. Most of this learning process happens when you intern and are able to stay late at night and go through the print and flat libraries of your employer to learn. But, not everyone is willing to intern or work for free. Here is the best book that I have found: Creative Fashion Design with Illustrator by Kevin Tallon. It puts all other books to shame.
The fastest way that I teach interns and new employees how to draw flats is by providing them with my workspace that I created to draw flats. I would love to show you what this looks like, but my computer is not allowing me to screen cap at this time.