Enacting the legislation means they agree that they are going to do it. The first 30 months of the "doing it" involves inventory of gold, etc, etc, so they can determine the correct value in gold to assign to the dollar. Then, after the 30 months, we start using gold backed currency.
There are certain things the President can do without consent of Congress. Go to the following address and you'll find the list of things that Trump has done since his inauguration. A good example of a VERY important "Executive Order" can be found on 12/21/2017