You don't. They fit to the unit carcase and the opening side is attached by sliders to the fridge door. The sliders should have been in a bag with the instructions on how to fit inside the machine. If they're not there, get onto your supplier.
If it's a flat plate with holes in it and a right angled piece with slides for the flat plate to go in you need to measure the thickness of the side panel of the housing and fix the flat plate that distance in from the edge of the decor door. Slide the right angled piece on, open the fridge door and line it up with the right angled piece and screw that to the door edge.
Before you drill or screw anything, use bluetack to hold the pieces in place and you'll understand how it works and be able to work it out from there.
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