First of all, you do not want the graphics card to be installed at this time.
Fit the i/o shield fully into the case.
It takes a bit of force and you need to go around the rim and press the shield fully into the case.
To get the motherboard to fit, Ideally you would want three hands, so a helper is good.
With the motherboard at a bit of an angle, push it into the shield slots.
It may take a bit of pressure to bend the tabs.
One hand keeps pressure on the end of the motherboard and lowers it onto the motherboard.
The other hands put the motherboard stand off screws in, one at a time.
Start with the corner nearest the shield. Thread, but do not tighten.
Using that as a pivot, place another standoff screw.
Keep them all lose and secure them. Do not tighten much or you may never get them off easily again. snug or less.
Lastly, if all else fails, your pc will work perfectly well without any i/o shield at all.