Prompted by this question:
Assume you care for a turtle or tortoise and by accident it falls from a height and cracks its shell.
The first and most important course of action is to see a vet as soon as possible! The shell can hide internal bleeding that can lead to death within minutes to days of the fall.
But assuming there is no vet in the area, or the vet isn't specialized in reptiles and cannot offer any advice, how should I care for my turtle / tortoise with a fractured shell?