Grab your tube of thermal paste, if you don't have one, I recommend Arctic MX-4. THERMAL PASTE IS NOT REUSABLE, YOU MUST REPLACE THE THERMAL PASTE IF YOU REMOVE THE HEATSINK, but the thermal pads (soft foam colored pads) are reusable.

Carefully line up the tube end with the center of the CPU, and put a small pea sized dot in the center. DO NOT SPREAD IT, THE HEATSINK WILL SPREAD IT.