The Brightness cutoff Detection mode is the best for most situations. Changing the Detection mode to Edge detection will give you an outline effect. You can try adjusting other parameters to see how they affect the result
Don't worry about getting right on the details that you're tracing at first. After you have traced close all the way around the detail, zoom in and fine tune the path to line up with the image. Holding the shift key while clicking on a point allows you to create a curve handle