How do I install IO shield?
You need to line up the shield and press firmly on the middle of all 4 edges, 1 at a time, until you hear/feel them click in place.
How do I push motherboard into IO shield?
Sometimes you have to bend the metal tabs that are attached from the top side of the cutouts so that they slide up over the port shrouds. From the outside of the case. The smaller side gets pushed into the case. Then you put the mobo on the stand-offs as you slide the I/O ports into the correct holes.
Do you need to install IO shield?
IO shield is IO shield, it protects the ports and to some extent the motherboard. It prevents dust from getting in, and I suppose it stabilizes the ports on the I/O to a small degree. You can make do without it, but you should probably get one when you can.
Do I bend the tabs on the IO shield?
The tabs are not essential. They are meant to bend backward to make a good grounding connection between the I/o panel and the motherboard. If the I/o panel fit is good you can leave the tabs where they are. That assumes you can properly plug in any other peripherals you need without interference.