When a new currency is created, how does it move from the central bank into circulation among people?

When a central bank creates new currency, it is sold off to the commercial (high-street) banks, a process known as seigniorage.

The commercial banks then distribute this money via their cash management division through all the channels they have, predominantly, ATMs and Over-the-Counter.

If the new currency in replacing old currency, then customers usually queue up at banks and exchange their old currency for the new one (happened in India very recently).