There is a tutorial in SW that will explain it. You need to bring up the 2d to 3d toolbar. Basically you bring in the 2d drawing and you start orientating corners so that it know's which view is the front and the right and the top. Then you can select geometry to extrude or to extrude cut and build the model. It's by far not automatic and I would only use it for pretty complex geometry. 99% of the time I just re-draw the model as its faster, and the geometry is more usable/changable over the 2d driven model.