The challenge in fitting a new serpentine belt(Gates FleetRunner K080830HD with a/c+intercooler) is that you need 3 hands. One hand is required to operate a 3/8" drive, 17" breaker bar($8 from Harbor Freight). The other two hands are required to slip the belt over the top of the alternator pulley. If the belt seems too short, check that it is not caught up on a protruding bolt head way down next to the crankshaft pulley.The offending bolt head is on the driver's side and you'll never see it from below or above -just fiddle and grope until the belt lies tight to the curve of the c/s pulley. If the belt seems too long, you forgot to make a routing diagram before you removed the old one ? The sticker on the lip of the hood showing the belt routing is a bit vague and faded ? Geno's has a good diagram if you did forget.
Before you fit the belt it is a good idea to spin the idler pulley to feel for roughness in the bearing. At 48,000 miles & the original belt my bearing has just a slight roughness so I'll fit a new pulley soon (when I replace the fan+fan clutch with an electric fan) Dayco 89005. It is worth checking the water pump pulley also.
To hold the end of the breaker bar so that the tensioner pulley is correctly positioned, I used a cheap ratchet strap . Make a few turns round the end of the breaker bar so the strap doesn't slide off. Wrap the other end around the convenient bracket on the passenger side fender wall -see photo-(if I can figure how to upload without an image host like Photobucket). The bracket has a sturdy bolt,wing nut and bracket on it -very convenient to loop the strap around. You can use the ratchet to get full movement of the tensioner Then you can use two hands to slip the new belt on the alternator pulley . It is then easy to check that all the grooves of the belt are correctly engaged in all the pulleys that have grooves. Don't forget to remove the ratchet strap and breaker bar before you start the engine!