The House sends out its Sergeant-at-Arms to arrest the defendant, he is tried on the spot, and the House decides whether to convict.
It is a little bit of a surprise to me but the Capital has a dungeon just for such purposes. And I find it interesting and very pleasing that:
…presidential pardons appear not to apply to civil contempt procedures such as inherent contempt because it is not an “offense against the United States” or an offense against “the dignity of public authority.”
I realize spending really needs to be cut but couldn’t we find enough money to enlarge the dungeon enough to hold a few more people? You would think that after spending a few days chained to the wall they would become more cooperative with Congressional investigations.