How did they do it?

With the help of 135 Republicans joining with the Democrats to pass something so clearly unconstitutional.

Absolutely disgusting.

It looks like they're trying to do the confiscations through the military.

American Military News explained more details:

A provision in the 2022 National Defense Authorization Act would allow military courts to issue protective orders that include "Red Flag" gun confiscation, according to the more than 1,300-page bill.

In the legislation, those beholden to the United States Code of Military Justice could be issued a "military court protective order" by a military judge or magistrate, which would make "possessing, receiving, or otherwise accessing a firearm" illegal.

"A military court protective order issued on an ex parte basis shall restrain a person from possessing, receiving, or otherwise accessing a firearm; and a military court protective order issued after the person to be subject to the order has received notice and opportunity to be heard on the order, shall restrain such person from possessing, receiving, or otherwise accessing a firearm in accordance with section 922 of title 18," SEC. 529 of H.R. 4350 states.