Orient the fan so its label is facing downward and the cable(s) is pointing towards the top of the computer.

If your fan has an arrow indicating airflow direction, make sure it's pointed towards the heatsink.

If you're using a different fan, orient it such that the fan is blowing towards the heatsink, not away.

Lay the fan on top of the heatsink, making sure the cables are routed so they poke out of the hole on the top of the computer.