Before we get started with how to make a pattern from a form, let's first ask the question, why? There are two main ways to make a pattern - flat pattern making, and draping.
Flat pattern making involves taking a basic flat pattern (sloper), and altering it to the desired style in flat paper form through cutting, measuring, and taping. An easy example to visualize is if you had a straight skirt, and wanted it to be A-Line. You would slash the skirt at regular intervals, and pivot the pieces apart from the waist until the desired level of flare is achieved. Then you would copy that over to a new paper pattern, and voila, new pattern.
Draping is patternmaking done on the form. Draping is useful for all kinds of patternmaking, but particularly for anything with draped elements (hence the name), like the two examples above. Cowls, ruching, asymmetry, and any elements that are hard to visualize in a flat fashion are where dress forms are truly indispensable. What I love about draping is that it is easier for me to visualize than flat pattern making, as creating a shape in 3D feels like a more organic process than drafting in 2D.
In this tutorial I am draping a simple form fitting pattern. While buying a flat pattern would probably be the quicker way to go in this case since it is an common shape, I wanted to keep it simple in order to demonstrate the basic process of draping, fitting, and drafting the pattern. All patterns, from a basic pattern like this to a complicated draped design, will follow very similar principles.