Refer to this link:
https://www.youtube.com/watch?v=dlEspgGGEt4
{PLEASE MAKE SURE POWER IS DISCONNECTED}
1. Remove the CPU fan (heatsink)
2. Remove thermal compound using rubbing alcohol (using a clean rag, paper towels/napkins will NOT do the job well), to get the correct amount, place the rag and cover the top of the bottle with the rag. Carefully turn over the bottle until it's inverted (upside-down) and turn it back over.
3. Without touching the sides, gently rub the top of the CPU until all thermal compound is gone
4. Apply your thermal compound, using the little-bubble technique (make a little bubble in the middle of the CPU, not a big one, very small) and spread the thermal paste out using a thermal compound spreader (should have come with your thermal paste). You don't need to get all of it, just spread it flat into a flat circle. (When heat is applied, it will naturally spread out)
5. Place the CPU fan (heatsink) back onto the CPU (correctly, otherwise this could ruin the thermal paste)