Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
I would like to congratulate you on your recent promotion to [name of new position].
Because of your hard work and dedication to the job, I know you'll succeed in your new role. Employees like you help [name of company] keep ahead of the competition and lead the way in the field of [name of field]. Your efforts are highly appreciated and respected.
Again, congratulations!
Sincerely yours,