You are done.
- Hold the middle button of the mouse (buy one if you are using a 2-button mouse...).
- Drag it to the right and to the bottom of the image. This defines the zoom area.
- Release the right button once you have selected the part
of the image you want to zoom to.
Just click with the right button of the mouse on the image.
To avoid zooming once you have defined a zoom area (assuming you have
changed your mind), just drag above the upper-left corner of the
selection rectangle. The selection rectangle disappears. Release the
mouse button: nothing happens!