We replaced mine last weekend and one task I have planned is a write-up on the job. It's a bit tricky but doable in-situ.
The special tools you need is a 3/8" square drive to retract the tensioner and a 6mm ball-ended Allen key to remove the lower spring bolt.
The main thing is to determine the tensioner is actually the problem in your case. Easiest way is to slip the auxillary belt off and to "exercise" the tensioner. If it's action is at all notchy, it needs replacement. If it is smooth then it's OK and your noise is caused by something else. With the auxillary belt off, it should be possible to isolate the guilty party. Top of the list after the tensioner is the aircon compressor.
Here is the retraction tool:
It was made from a socket set short extension with two flats ground to enable a 15mm spanner to fit on the end. It was also necessary to grind about 2mm off its length to give room to get it into the tensioner.
Here is the tool in use. Engine and other impediments removed for clarity.
This
thread may also help. A picture near the end shows how the tensioner fails.
I'll get the tensioner replacement guide written ASAP.