You may need to remove the motherboard from the case unless there is a hole in the tray the motherboard is mounted on.
(so you can see the pins clearly)
It is vital all four of the mounting pins go through the holes in the motherboard and the push pins expand the ends of the mounting pins
Looking at a brand new heatsink (not yet fitted to a PC) will give you the correct start position for the mounting pins
The video on youtube should explain it clearly for you.
best of luck