These are the easy instructions because the ones on jdownloads.com documents are really complex.

This assumes you have not already messed around with permissions. If you have change everything back to inherited.

1. Goto JDownloads admin and click on Options at the top right.
2. Click on Public and change the Download permission to Denied.
3. Click on Registered and change the Download permission to Allowed.
4. Click Save & Close. Permissions will now cascade down
5. Done

Notes

• You can override individual download items permissions downstream by edit their permissions individually.
• Not 100% related - The permissions set within JDownloads for the daily limits might not cascade down so have to be set for every group.