CPU (Central Prossing Unit):
When placing the CPU on the motherboard make sure that the brown leaver is up and on the CUP there will be a pin missing make sure that the missing pin is lined up without a hole on the ZIF (Zero Insertion Force) socket
Hard Drive:
When placing the hard drive into the computer case make sure it is the right way up by making sure that the sockets are at the bottom and there twist the green locks so it will not fall out and the colour may vary in different cases
Inserting ATX Power Cable In To The Motherboard:
When placing the ATX Power cable into the motherboard make sure that you place the cable in the correct slot which is the slot next to the RAM slots which should be white slot