Print everything without supports in its given orientation except bone_print_with_supports.stl. You can superglue bone_print_with_supports.stl to jaw_upper.stl. Cut 1.75mm filament to use as a pin to hold jaw_upper.stl and jaw_lower.stl together.

https://www.youtube.com/watch?v=6aXmk-A7hyo

Note that the bone is not supposed to spin internally despite the backpack being turned 180 degrees. THE BONE IS ALWAYS UPRIGHT. In Crewmate mode, the backpack locks the bone+jaw assembly to the bottom half, so when you lift the upper half the bone does not follow. In Impostor mode, the bone is still inside the helmet/upper half, just that the backpack locks the bone+jaw assembly to the upper half.

Note that jaw_upper.stl is not supposed to rotate when you rotate the backpack_upper.stl. If it does, you might have to sand down the pin in jaw_upper.stl.

Update 17 Nov 2020:
jaw_lower_soft.stl and jaw_upper_soft.stl added for soft-tipped teeth. backpack_lower_snap.stl and backpack_upper_snap.stl added for a tighter backpack which clicks together.