If Snapshot is not running, choose Snapshot from the Image Viewer File menu.
Click the Snap button.
The mouse pointer becomes a plus sign.
Move the mouse pointer to a corner of the region to be snapped, press and hold mouse button 1, drag the mouse diagonally to encompass the region to be snapped, and release mouse button 1.
The message at the bottom of the window reads "Snap succeeded" once the region is snapped.