I used a 3/4" socket (I think) on a breaker bar. I took the spark plug wires off and had a friend crank the engine. The breaker bar went up against the suspension and then the bolt came right out. You don't need an impact wrench. Putting it back on tight isn't as easy. I'd recommend some blue thread lock compound on the threads because no way you'll get it torqued back to 85 foot pounds. I ran my UDP HARD at the autocross and it didn't come loose.
To loosen the tensioner, remove the belt shield (two 10mm bolts). Then push upwards with your hand on the tensioner or get on the 15mm bolt in the center of the tensioner and try to "tighten" it while under the car. While the tensioner is retracted, pull the belt off from the A/C or water pump pulley, then then remove the belt by hand. When replacing it, loop it around all the pulleys except the A/C pulley. Again, retract the tensioner, and then loop the belt over the A/C pulley.a few things to note:
1) Before starting, support the car with a jackstand.
2)Remove the passenger side wheel.
3) Using a 10MM socket, take out the one bolt holding your coolant resevoir. Unhook it from it's mounting spot and move it out of the way.
4) break the power steering fluid container free from it's mounting bracket and slide it up and out of it's holders and move it out of the way. This will help you get to the tensioner pulley from the topside.
5) snap off the plastic walls of the little insert behind the underdrive pulley where the belt will go. otherwise it will rub on this piece, make lots o' noise, and ruin your accessory belt. If you want to take it off the engine to work with it better, it's held on with two 3/8" bolts. I think that's equivalent to 8mm. But I'm not sure.
Here is a Belt Diagram: