I wrote a post on this subject a while back:
It should be safe if caution and a smooth belt without studs or buckles is used. Ensure the smacks are delivered slowly and the spanker doesn’t use too much force. The best way to achieve this is for the recipient to always be in control, and able to dictate the pace and the force of the impact.
“Please Sir, I deserve another.”
And don’t aim for the tenderest regions right away, the mound is a good target to smack initially.
Done right, it can be extremely erotic, just don’t spoil the moment by escalating too quickly.