Clean both the CPU top and the heatsink bottom by slightly moistening a paper towel with rubbing alcohol and rubbing lightly.

Grab your thermal paste and put a pea-sized amount in the middle of the CPU top.

Gently replace the heatsink on top of the CPU and re-lock the heatsink fasteners.

Replug the CPU Fan connector to the motherboard header.

Check out the How to Apply Thermal Paste guide for more details and best practices.