If you have screen vice clamps, use them to work around the camera bar.

Lay your phone screen side down on a clean, flat surface.

Put tape around the perimeter of your phone to protect the rear cover.

Place coins or similarly thick objects around the perimeter of the back cover. Make sure they're as thick or thicker than the camera bump.

Place heavy objects such as books on your phone and leave them for 30 minutes or longer. Ideally, leave them stacked overnight.