These can be especially fiddly, but you need at least the power button connected in order to turn your PC on.
Connect the case plugs and buttons to the motherboard. A double-wide row of pins — the location of which will be noted in your manual — handles the buttons for reset and power, and activity LEDs for power and storage. These can be especially fiddly, but you need at least the power button connected in order to turn your PC on.