After screwing up the motherboard with the metallic plate, you have to install CPU and memory in the slots available with the motherboard. Only after completing installation of these components, you can close the computer case.
The motherboard will generally have a plastic socket covering CPU slot. You have to unlock the slot by simply pulling out the plastic wrap. Then align the CPU to the socket provided. To align the CPU, look for corner that has diagonal section containing the CPU label. Your motherboard will also have a similar pattern in the slot and you can now align the CPU. You should take care that you don’t push the CPU hard. The CPU should fall gently into the place if you have properly aligned it.
To lock the CPU, lower and lock the socket arm. This will hold your CPU in the right position on the motherboard. Then you have to install the heatsink on the top of CPU. Most of the fans come with thermal pad to prevent friction and heat. The CPU socket will have places to which the clamps of the fan have to be attached. Using the locking arm in the fan, you can easily fit that on the top of CPU. Then connect the power plug to the motherboard in the corresponding socket which can be found from the manual.