They want you to make sure the valve is all the way closed to adjust or shim them. Just take the valve cover off (you have to anyway) and turn the engine by hand untill you see them closed or all the way up.
What they want is to make sure the cam lobe is at it's lowest point. One way to ensure that is to rotate engine until valve is open all the way, then rotate engine one full revolution. That will put the lifter oposite the highest point, which is the lowest.