Well, if he is elected President, then he will do it the way any other president would, by ordering the employees of USCIS and the State Department to implement the order.
If visas are not being granted to a group of people, then they aren't going to be able to arrive in the US by airplane or ship. Have you ever tried to board an airplane without a suitable visa for the destination country? It's not going to happen. If it does then the arriving passenger won't be able to get through immigration, and they'll be stuck in the international area of the airport.
For people arriving by car, bus, or train, the border patrol agents aren't about to let someone enter the country without a suitable visa.
Of course some people may sneak across the border from Mexico or Canada, but I think Mr. Trump intends to make that a lot harder to do than it is now.
I saw that movie.
Residents of thirty eight countries don't require visas to enter the U.S., That includes France, Belgium, Great Britain, Germany and many other countries with significant Muslim populations. So most of those responsible for the Paris massacres could enter the U.S. without any background check.