As Alan said, you need to open up the hole at the top. More than one way to do this, I usually just use a sharp blade. However you do it though, as the model is already covered make sure that you cut away the film first so it doesn't snag when you open the hole up.
Also you must make sure you lubricate the knuckle as Alan said. It is also best to install them all to one side first, make sure they are lined up correctly then let the glue set before bringing the 2 parts together, this ensures that the hinge line is straight or you will end up with a stiff hinge.
edit. Was typing that while Mike was posting, clearly I am on the side of one surface at a time, trying to do it all in one go is a big risk and is likely to end in tears!!
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