For starters, it is a nice thing to do. Everyone likes getting some praise. When it is your turn to advance at work, you will appreciate hearing well wishes from your co-workers. Besides, it is a relationship builder.
Tips on how to do it:
1. Keep the email or letter strictly professional.
2. Don't wait too long. As soon as the promotion is made public, feel free to send a congratulatory email or letter.
3. In your email or letter, explain how you hear the news.
4. Express praise and approval. Say something like you think they are the right person for this job and nobody can do better.
Dear John,
Congratulations on your promotion to Manager of the Sales Department. I heard about your well-deserved promotion through our HR manager. You have done a fine job in sales for many years, and deserve the recognition of the new position.
Best wishes for continued success in your career.
Sincerely,
Mary