As Gary says, what you need is a large light source.
There is a book,
Light Science and Magic that will explain it all in amazing detail. I have seen this in the Dymocks in Sydney. as well as all the usual Web places.
The light source has to appear large to the subject, not be large in any absolute sense. The sun is the largest absolute light source we have, but because it is so far away, it appears small, so it produces harsh light. Add clouds however, and the light source goes horizon to horizon. Much larger, so the light source appears larger & the shadows suddenly become soft. OTOH, A bare flash can appear large to a very small object IF the flash is very close to it.
The simplest method in practice may be to simply take the
model outside on a cloudy day. If it has to stay indoors, you could try bouncing the flash off the walls or ceiling. If they are coloured, or too far away, you could use a (white) sheet as a reflector.