This concept is beautifully brought to life in the book Chimpanzee Politics.
Often in life you plan to do, achieve or become something; and tell yourself that, when you do, you'll behave accordingly.
However, the sequencing is opposite. If you want to be selected or appointed as a leader, start leading. Start dressing like a leader, talking, thinking and acting like a leader. You are more likely to achieve what you want once your behaviour matches that outcome.
If you want to be a writer, you are not a writer once your first book is published. You are a writer once you start thinking, acting, and referring to yourself as a writer.
If you want to become a musician you are one. Maybe the worst musician in the whole world, who hasn't even decided what instrument to play, let alone touched one; but a musician nonetheless; who is now hopefully on a journey of improvement.
The underlying point is that, more often than not, whatever the achievement, the first validation is your state of mind; and further validations (eg actually getting that promotion), are more likely to follow once that first validation is awarded - to you by you.
Terry Crews, former NFL player and television star, talks about this as being a key principle for him in the excellent Tim Ferris podcast here.
It is of course a variant of the old adage "dress for the job you want, not the job you have". But it's about a lot more than getting a job.
So whatever you decide to do or become, give yourself the license to become it immediately. Be it in your head, and your actions. The rest will follow.