Short answer
Start before the promotion. Give them a small piece of real leadership first, like running a project across two teams or onboarding a new hire. Train them on feedback, delegation and decision-making. After the promotion, pair the training with a senior leader who checks in on real situations, not just a calendar slot. The goal is that their first hard conversation isn’t their first try.
The longer answer
Promotion waves are risky. A company that promotes several strong individual contributors at once can lose some of its best workers and gain some struggling managers in the same month.
Two questions help. Does this person want to lead people, or do they want the title and pay? And have they had any chance to practice? If the answer to the second is no, build that chance in before the move.