I'm assuming you are dealing with the aux belt here.
First you need to remove the off side headlight (for access).
Just below the power steering pump pulley you will find the tensioner pulley.
You will need a 7mm allen (hex) bit that idealy you can fit a spanner over the back of, (it needs to be short to fit in the limited space) and a 22mm spanner.
Using the 7mm hex bit, losen (don't undo) the bolt in the middle of the pulley. It is important to leave it tight enough so that the pulley doesnt slip any teeth when tightening, but lose enough so that the pulley can be adjusted.
You can then place the 22mm spanner over the nut on the pulley to tighten or losen the belt.
Turn anti-clockwise to tighten the belt.
(should be about 2mm of play between the power steering pump and alternator).
Once it is tensioned correctly, re-tighten the allen bolt in the centre of the pully to hold it in possition.
Start the engine and make sure it stays tight before you put the headlight back.
Job done!