Improper installation of the casings back cover can lead to damage of the actual frame structure which will result in the back cover not holding properly against the weight of the iPhone X. Please follow these instructions to ensure a proper install.
Unlock the back cover
Press the black button to release the lock mechanism from the back cover. Gently push the cover out as you're holding the button down. You will notice the cover will open and will look like something in step two
Slide the cover out
As you can see in the picture the cover is lifted from the lock mechanism. Pull it out gently from the bottom lock which will be displayed below. You have to pull towards the lock button and away from the bottom
Examine the housing
Prior to placing your device inside the bumper, notice the special ridges created for the bottom lock. Make sure the teeth of the cover go under the silver trim of the bumper. Place your device and follow the next step
Do VS. DO NOT
Once you have placed your device inside the bumper and understood how the bottom lock works, proceed to slide the back cover in place. DO NOT simply lay it on top and lock it in place, DO slide the cover in and once you see the black teeth from the back cover under apply pressure on top (near the lock) to close it in place.