After you select the "Create Custom Crop" under the "Edit" header, in the resulting popup screen set the ratio that you want, and type in the desired width (in pixels) in the second column. Make sure the width is less than the total width of your to-be-cropped image. At the bottom of the popup screen, select "Apply To Image", and a crop square in the correct ratio will appear in the upper left corner of your image. Now the key point is:

To Move the crop square, slide your cursor into the square, hold down the RIGHT mouse button and drag the square to center on the approximate area of your crop.

To Adjust the Size of the crop box, move your cursor to the left or right borders of the crop box until the double-headed arrow appears, then hold the Ctrl Key down while you drag the line by holding the LEFT mouse button. Fine tune the position of the box with the RIGHT Mouse button dragging procedure.