It's a pretty simple job, get the right hand front of the car up securely on an axle stand, remove the wheel and take off the belt cover that clips on underneath. Gently loosen the 3 bolts holding the power steering pump on, then with a spanner ready to retighten one of the pump bolts, turn the "geared" looking bolt to tighten up the belt and nip up the pump bolt. Once you're happy with the tension, do the pump bolts back up, put the cover back on and stick your wheel back on.
If you need to replace the belt, the process as above still applies but you'll just need to swap the old belt for the new by releasing the belt tension first, pull off old belt, put on new belt making sure it's located on the pulleys properly and tension it.
A new belt will cost you about £4.
