Hi Monika, another tip along with Bill's explanation above. If you are having difficulty 'seeing' where the actual 'graduation' is, when sliding the grad filters up and down is with your '3rd' hand (
) - depress the DOF button to stop down the lens/aperture so you can actually see the darkness. Because you are using a Canon, it shall be of the left-hand side on the front of the camera as you look through the view-finder. It is a small dome button next to the lens on the camera face.
Hope this helps
Wilko