The first thing you should do is unpack your ATX case. Take off the cover of your case so that you can access the inside. Place the case on a desk so that you are looking down towards the open case. Your case should come with motherboard mounting screws. If your ATX back plate it not already fitted you can fit it by placing your plate near the ATX back plate cut out and pushing the plate outwards, it should clip on.

Now place your motherboard on top of the mounting screw holes. Make sure your ATX devices on the motherboard such as PS/2 and parallel port are facing towards ATX back plate cut out. Gently push your motherboard towards the cut out, every devices should fit easily into its corresponding cut out, as shown below.

The screw holes on your motherboard should align with the screw holes on your case. Place your screws that came with the case into the appropriate holes and gently screw it on using a screw driver.

The motherboard is now securely mounted to the case. You can now place the ATX power connector to the motherboard. Your ATX case should come with a power supply unit (PSU) and should already be mounted to the case. The ATX power connector is shown on image below.

Place the ATX power connector on top of the power socket on the motherboard. Push down the power connector and it should clip onto the socket. If you try to fit the power connector the wrong way round, it won't fit, it will only fit one way. So, if the power connector does not go in, it should go in the other way round.