Take the CPU ATX cabinet and place it horizontally on the soft floor as shown in the image.
Take the SMPS (Power Supply) and place it in the cabinet as shown in the image.
The SMPS has to be fixed with screws provided along with ATX cabinet at the back side of the cabinet.
After fixing the SMPS, take the motherboard and place it in cabinet such a way that all ports (keyboard, mouse, USB, Printer, sound, etc) are placed at the small rectangle opening at the back side of the cabinet. That is the motherboard has to be placed just below the SMPS.
Now the motherboard has to be fixed to the cabinet using screws as shown below.
There are small holes provided in the motherboard for fixing the motherboard to the cabinet with the help of screws. After fixing the motherboard screws make sure your motherboard is not shaking.
Note: Do not screw the motherboard to the cabinet too hard or too tight. Because there are chances that backside of the motherboard getting contact with the metal portion of the cabinet making your computer getting restarted very frequently.