Once you have the crown isolated from it's background.
1. Make a new layer below the crown layer, make sure it's selected/highlighted.
2. Cmd + click the crown layers thumbnail to make a selection.
3. Fill the selection with black.
4. Convert to a Smart Object (right click layer then select). We do this so we can edit the shadow if we need to.
5. Use the Move Tool to position the shadow.
6. Add a smart "Gaussian Blur" filter (Go to >Filter >Blur >Gaussian Blur) adjust to your liking.
To edit, just double click the actual word "Gaussian Blur" below the smart filter mask.
7. Use the Opacity slider of the shadow layer to adjust the transparency of the shadow.
You now have full control over the shadow.
(Note: I made this example demo very fast, it's not perfect. You take your time)
Before
After
Tight shadow...
Loose shadow