Mobile users click here to view diagram.
The diagram provided shows both with and without A/C.
Rotate the tensioner pulley enough to remove the old belt from over the pulley.
Inspect all pulleys and make sure there is no debris or oil on them.
Re-route your new belt leaving the tensioner pulley for last.
Rotate the tensioner pulley enough to slide the belt over it.
Inspect all pulleys and make sure the belt is lined up correctly on each one.
Resources:
Basics
Read about tensioner pulleys.
How to check your belt