I'm assuming you have a pretty standard setup.
It's a LOT easier if you remove the I/C piping where it turns the corner to head off down through the skirt.
Now, loosen the pivot bolt. You should be able to tell it's the pivot. It has a nut on one end - you will need to turn the P/S pulley so you can stick a socket through the hole on the pulley. To adjust the tension, there is a bolt on the underside (12mm head, IIRC) - hence the need to remove the I/C piping. Screw the bolt "in" to tension the belt . Tighten the pivot bolt. Replace the I/C piping.