Threads Magazine is currently hosting a very useful article on Fashion Sketching for Untrained Artists . Jennifer Sauer covers some key tips on how to make visual notations of what you see and create sketches of garments.
As anyone who has done any sewing knows, your figure type will dictate the look of the finished garment. In order to develop a sense of what a garment might look like on a particular figure fashion designers use croquis which are outlines of various figure types. The article even provides a number of these in pdf format for readers to download.