If they were going to do it here in the United States, they'd take you by surprise and without prior warning. A few hundred boatloads of UN troops would secretly arrive at a U.S. port and be secretly dispersed throughout the country. They'll come to your home at some early hour, break down the door and conduct a search. The whole country-wide seizure will be lightning quick and over very fast, otherwise the people would be able to organize and resist with force.
If you're wondering how they'll be able to tell who owns firearms and who doesn't... well, they have their ways. Carnivore and Echelon are two that come to mind, but even IP tracing will work. You send an E-Mail about a gun you own, make a post here on ARF.com, use a credit card to buy a weapon, ammo, mags or attachments -- that digital record doesn't just dissapear. It's there forever. Everything you've ever done digitally is stored on some machine in some basement, and if they want it they will find a way to get it.
And if you think the then-President is actually going to announce the arrival of UN troops to take away our guns, and that you'll have the luxury of getting 5 or 6 friends together and armed before they arrive at your doorstep, you're seriously underestimating the powers that be. For all the hate the Clintons and various gun-hating liberals get, they aren't stupid.
That all being said, this is very unlikely to happen. Any president who took away our guns with force would be unseated from power. Likewise, any Congressperson who didn't move to take action immediately would also feel the wrath of the electorate.
I think the more likely option would be Congress and the states using lies and misinformation on an uninformed public to gradually increase the amount of gun control until the 2nd Amendment basically ceased to exist. This more peaceful method is preferable because it'd have widespread public support (like the AWB did). UN troops on American soil, on the other hand, would just be a catalyst for disaster.