You’ll have to use some force to snap all four corners into place.
The IO (input / output) shield, which covers the area around your rear ports, comes with your motherboard. You’ll need to fit the shield into the chassis before you install the motherboard itself, making sure it's the right way up, so that your motherboard's external ports will fit through the holes once both are installed. You’ll have to use some force to snap all four corners into place. Be careful of sharp edges (that’s why you have the band-aids) as well as metal bits that can block the ports--especially if you have a budget motherboard.