Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 245

Figure 3.62
Zoom into your image, enlarging the picture. Follow the contour of the curved glass by breaking it up in stages when setting the con-
trol points. The dark outline is the contour that has been applied to the glass so far. The selection has not been closed yet.
Next, start at a distinctive spot on the image and set the first control
point with a left mouse click. In the end, you must return to the first control
point. Follow the contour of the wine glass with the tool, applying the rubber
band to the contour. Then click again to set another control point. A line now
connects the control points that you have set. If your image has a curve, you
must set several control points, splitting up the curve in a polygon. If there is
less of a curve, you can reduce the amount of control points, drawing out the
length of the line between the points. Continue the procedure until you have
circumnavigated the entire figure. At the end, connect the selection with the
first control point to extract your figure.
Basically that's it. You can continue editing the extracted selection as you
would any other selection in the Select menu.
Now you will continue editing the image. First you should save the
extracted figure by choosing Select > Save to Channel . Then you should feather
your selection ( Select > Feather ) with a gradient of 3 pixels.