You will need a half inch drive breaker bar or ratchet insert it into the tensioner there is a square cut out for the half inch shaft of your breaker bar then you can use it to hold the tension and install your new belt, there should be a routing diagram in front of or on the radiator.

I only know of 2 methods that are commonly used to ensure that the serpentine belt stays tight. You mention a tensioner, which seems to be the most common way with modern vehicles. I have done a few on various models, including Ford, Mercury & Toyota. I will describe the process for these. Yours should be the same. 1st, it is extremely important that you purchase the correct size belt for this (or any other) vehicle. Next, find the proper size socket to fit on the bolt to the tensioner. Place the socket on a ratchet. Find a piece of pipe that is 18-24" long and will fit over the handle of the ratchet, giving you a longer handle, and therefore, more leverage. Set the ratchet to TIGHTEN the bolt. Place the socket on the bolt with the ratchet attached. Slide the pipe over the ratchet handle. Holding the furtheset end of the pipe, turn the bolt CLOCKWISE. The bolt will not loosen. The tensioner will move, providing slack for the serpentine belt. Slip the belt off of the tensioner. LOOK CAREFULLY at the way that the belt is routed around the various pulleys and harmonic balancer before removing the old belt. There is generally a diagram of the routing pattern on a sticker under the hood. Installation is the reverse of removal. Route the belt around all the pulleys, leaving the tensioner for last. Again, turn the tensioner bolt clockwise to provide slack. Slide the belt over the tensioner. Remove the Pipe / Ratchet / socket combination. Dispose of old belt. The only other way that I know of is on cars without a tensioner. In this instance, the alterantor has a bracket with a lot slot in it for tensioning manually. Loosen the 2 bolts to the alternator and provide slack to remove the belt. Install the new belt, following proper routing. Use a pry bar to leverage the alternator into the position that will make the belt snug. Tighten the alternator adjusting bolt. Dispose of old belt. Both methods are very easy- as long as you use a ratchet with a very long handle or place a pipe over the ratchet handle to achieve leverage. Good luck.

Look at this and see if it helps. http://autorepair.about.com/library/weekly/aa071205b.htm or here is videos http://www.expertvillage.com/video/11102_serpentine-belts-removing-using-jacking-screw.htm ***Try turning the bolt clockwise.