Look at the new belt in comparison to the old one to be sure they are the same width and have the same number of ribs going across the belt horizontally.  The old belt may be slightly longer than the new one due to stretching.  If the new belt is not the same width as the old one, it is the wrong belt. Make sure the new belt doesn't show any signs of damage from the factory or it may tear after you install it. Run the new belt through the pulleys in exactly the same way the old one was.  Refer to the belt diagram in your vehicle's owner's manual or the picture you took of the belt before removing it to guide you.  The belt should be long enough to route around all of the pulleys except the tensioner until you relieve the pressure on it. If the belt is loose after routing it, it is either the wrong size belt or you've routed it incorrectly. Use the same wrench to loosen the auto tensioner pulley, then pull the belt over it.  Once the belt is in place, release the pressure on the tensioner so it holds the belt in place. It may be difficult to apply pressure to the tensioner and route the belt, so you may want to ask a friend to help. If your vehicle does not have an auto tensioner, insert a pry bar or piece of wood between the alternator and the engine.  Run the belt over the alternator pulley, then use the bar to pry the alternator away from the engine and increase the tension on the belt.  Tighten the bolts again as you apply tension.  You may need help tightening the bolts as you apply pressure to the alternator. Be careful not to pry against anything plastic in your engine bay to avoid breaking anything.

Summary:
Compare the new belt to the old one. Route the belt through the pulleys. Apply pressure on the auto tensioner and put the belt over it. Increase the tension on the belt.