How exactly does 'printing money' work?

http://baltimorecheckbook.tumblr.com/post/144209132052/how-exactly-does-printing-money-work#_=_

Donald Trump says don’t worry, America will always be able to pay its debts because the government can simply print more money.



from http://www.wbaltv.com/money/how-exactly-does-printing-money-work/39491794