Disconnect power from the system unit of your personal computer. To mount the power supply to the motherboard, use a single 20 or 24 pin connector. Wear it wrong you will not succeed, because it comes with locking keys. If you intend to use a PSU with 24 pins, and the motherboard there are only 20, that's fine. Not only will involve the contacts 11, 12, 23 and 24. If we consider the opposite case, i.e., connecting 20 pin power supply to 24 pin motherboard, this connection is not possible.
Check on the motherboard having four additional contacts, to connect an atx power supply. These motherboards are designed to use powerful processors. To the processor suitable additional four wires: 2 black and 2 yellow. Black wire with zero potential, and the yellow wire voltage is +12V If the power supply is ATX does not have a corresponding connector of this motherboard, it use to connect is impossible.
Use the special atx power supply, if there is a need to connect a powerful graphics card that requires additional power. Note that the connectors on a standard power supply and this graphics card is very similar, but different set of stresses, and sometimes the configuration keys that will save you from incorrect connection.
In turn connect to the power supply first motherboard, then the processor and graphics card, if there is such a need. The characteristics of the power supply must be specified, for which it is intended and the number of connections is designed.
Also consider the power supply, because even if it is to approach the model and the connectors may not be enough power, to power all elements of the system unit.