To evenly compress your iPhone and strengthen the bond of the back glass to the frame, you need to account for the camera bump.

If you have the box your iPhone came in, take the lid and place it on a flat surface.

If you don't have your original box, skip this step for an alternate method.

Place your iPhone screen-side-up in the lid of your box with the camera bump in its recess.

Find something close to the size of your iPhone, but taller than the sides of the box. Stack it on top of the iPhone, followed by a few heavy objects.

Leave the objects in place for at least thirty minutes. The lighter the objects, the longer they should be left. Ideally, leave them stacked overnight.