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

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.