The stellation diagram is just a matter of projective or descriptive geometrical construction; choosing one reference face plane and extending the other faces until their planes intersect.
Once you have that, a method is described by Coxeter and du Val respectively in _The Fifty-Nine Icosahedra_ , which has been republished a few times.
Coxeter identified faces of the stellations based on the combinatorics of the edges in the diagram, then du Val used that to identify the spatial cells thus created.