The CPU Fan Assembly:
Remove the CPU fan from the box it came in or if you bought one separate get it out now along with the instructions that came with the fan for assembly.
When buying a separate CPU fan you must be careful to follow the CPU manufacturers recommendation for fans.
Some manufactures will void the warranty unless you use the CPU fan that came with the CPU or buy one that they recommend.
Whichever fan you have there should be some heat-sink material in the box in one of three forms.
In the box there might be a small gelatin capsule or a small hypo with grease in it, or the more common use nowadays what looks like a piece of tape on the underside of the CPU fan.
If you got the capsule break it open (usually they just pull apart) and squeeze the contents onto the center of the smooth metal surface of the bottom of the CPU fan. If you got the hypo then open the tip or cut off the tip (whichever method is needed for that hypo) and squeeze an amount equal to about a large drop of water from a faucet onto the center of the metal surface of the fan bottom.
If there is a tape on the bottom of the CPU fan: (which is most likely nowadays)
Looking carefully you will see that it has a very thin layer of what appears to be plastic as the top layer, much like the top protective layer you will find on some more expensive two sided tapes you can buy to adhere small objects to walls.
Peel the thin layer off the top leaving the small ribbon of grease on the fan
Be careful just to peel off the very thin Protective film that covers the ribbon of grease which keeps the grease from being smeared when the CPU fan is being handled.
The last fan I bought didn't have a thin cover on the tape style heat-sink and I almost peeled off the ribbon of grease that was there.
So look carefully before you start removing anything.
The substance your putting on the flat metal surface of the bottom of the fan is not really a grease. (but is called that by everyone I have meet or talked to)
It is actually a heat transfer substance that quickly moves heat from the CPU to the CPU fan heat sink so the fan can dissipate it before the CPU can burn itself out.(if its working properly that is) computer assembly
After the heat sink has been placed on the metal or the tape has been removed revealing the heat sink underneath, place the CPU fan on top of the CPU centering it as best as you can.
Look at the diagram that will come with the fan in the installation instructions to make sure you have the locking mechanisms aligned correctly with the area of the socket on the motherboard that the CPU fan is supposed to latch onto.
Some of these will only fit in one direction properly and once you have the heat-sink material bonded to the CPU fan you don't want to replace it as this will cause the bond to break and may hinder the heat transfer process.
No intention here to worry you, I have misaligned a few in my past and had no problems occur because of it.
I just want you to know that the fewer times you move the fan and CPU after applying the heat-sink material the better the bond will be.
After placing the CPU fan on the CPU, lightly press down on the fan and slowly rotate it about a quarter of a turn to the right then left and bring it back to the centered position.
This is to help spread the heat sink evenly over the CPU so it transfers heat more efficiently and will create a tight bond between the CPU fan heat sink and the CPU.
Do not pick the fan back up off the CPU after placing it or slide it around to spread the heat sink, the simple rotating motion is all that is needed and you might break the bond you created making the heat transfer less effective.
Now follow the manufacturers assembly instructions to latch the CPU fan into place.
Although the hooks on the CPU sockets are the same on motherboards that take the same style of CPU the actual method will vary from fan to fan.
Some use a lever system to latch. Some use spring clamps and others will simply press into place. So this step I can only say to fully read the included instructions and carefully follow them to the letter.
No matter which style of locking mechanism your CPU fan has, read the instructions that was packaged with your CPU fan and follow the assembly instructions carefully.
After the fan has been latched down take the power wires coming from the fan assembly and locate the nearest plug in for it.