I cut out a short section of a paperclip to use as the hinge, but you could also try the included hinge pin. Once the stand is installed in the camera protector, that goes on the phone and is held in place underneath the case.
Also see the version of the case without a stand:
https://www.printables.com/model/1016113-iphone-16-pro-slim-case
The author marked this model as their own original creation.