Both the IS 40 and IS 60 are installed in a similar fashion. What I would personally recommend doing first is attaching the mounting brackets to the cooler itself. Next, you will need to find the placement you will need for the screws, depending on your CPU socket type, put them in place then use the thumb screws to secure them.
Now it is time to apply the thermal paste, set the cooler atop the CPU and screw the nuts onto the previously installed screws that should now be poking through the back side of the motherboard. .