1) Something to obstruct the light with, like dense clouds, trees, or a terrain ridge.
2) An atmospheric element like cloud or hazy atmosphere to cast shadows into, since god rays are actually shadows technically.
This usually goes a bit easier if you have a dense atmosphere or layer of haze/cloud to cast your shadows into.
Setting "receive shadows from surfaces" *can* make it easier, but is definitely not a strict condition for getting god rays.
Be aware that a once obtained god ray effect can be lost upon enabling soft shadows.
