Put two dots above the (smallest increment of the) repeating part of the pattern. Work on getting those two dots to overlay (forming three dots with the one in the center bright and those on the outside lighter.)

Once you can do that it's as easy as moving your eyes down while maintaining your eye position (and if that really doesn't work you can try drawing two dark black lines down the image.)

Enjoy! Unless you can't get those dots to overlay in which case you at least then know why you were never able to do it :)