To crop a picture, hover over (or click) the crop icon to display the crop dialog. Now move the mouse pointer to the approximate top left of the area you wish to crop. Click the left mouse button and drag down and to the right to create a crop rectangle.
You now a pending crop rectangle. It's called 'pending' because you have not actually cropped the image yet. You can now adjust the size and position of the crop rectangle by dragging the sides or corners of the crop rectangle.
To remove the pending crop, click the Clear button.
To crop the image click the Crop button
See also
Un-crop
Rule of Thirds
Aspect ratio