Step 3: The processor cooler
Before you place the CPU cooler, the processor needs a layer of thermal paste to ensure optimal heat conductivity. Many coolers come with this layer applied already. It is easy to see, if the cooler is metal on the bottom you need to add the thermal paste. If you see a sticky layer, then you don't need to add any paste.
Thermal paste generally comes in a tube. Drop a pea-sized amount in the middle of the processor. Spread it out equally over the surface area of the processor, either with the tube or with a piece of cardboard. Please note that more is not better with thermal paste. The goal is to make the layer as thin as possible. Also make sure no paste lands next to the processor. Some varieties, especially the silver ones, conduct electricity which can cause a short circuit. If you get paste on your skin you should wash it off immediately as it is quite toxic.
After you have applied thermal paste, you can mount the cooler. Basic Intel coolers such as those included with processors work with a click system. Turn the four push pins on the cooler so the notches point inward. Now click the cooler into the four holes around the socket and fasten it by pressing the push pins down. Standard AMD coolers use a clamp that you fasten on two sides to a bracket around the socket. The cooler is locked into place by a lever.
More fancy and heavy coolers usually come with a backplate that needs to be attached to the back of the motherboard, on which the cooler is then fastened. The attachment type varies from cooler to cooler, so please refer to the included manual.
After attaching and securing the cooler you have to connect the cable from the fan to the CPU fan connection on the motherboard. Usually this is the only four-pin FAN connection in the vicinity of the processor socket. If the cable is really long, you can add a loop to make it more compact.