A badly adjusted or loose alternator belt can cause overheating of the belt, squealing when first starting or pulling away, and , critically on an F/TF, can fail, jamming the timing belt and wrecking the engine.
Firstly, jack up the car so that you can gain access from underneath. Remember to chock the front wheels and use axle stands for safety.
Remove the offside rear wheel and you will see a plastic cover between you and the engine. This is held on with about 3 plastic crosshead screws. Remove these and remove the plastic panel.
The F has a slightly different cover, or may not have one at all.
This will now give you access to the crankshaft pulley, alternator belt the two of the bolts that you need to undo.
You can also just see the alternator through the subframe
Next, loosen the bolt that holds the adjuster bracket to the engine, from the wheel arch.
Loosen the lower alternator securing bolt.
The alternator is now loose on the 2 lower securing bolts. You should now tighten or slacken the 8mm adjusting bolt, shown here on a removed engine (no way to get the camera in on the car), until the correct tension is felt on the belt. This is best done from underneath the car with a ratchet and extension.
I went for about 15mm movement up/down at the longest point. You can also twist the belt to get a feeling of how tight it is. There is probably a correct figure somewhere, but I have done a few belts in my time and tend to go by feel, which is difficult to put in words. Over tightening can cause more problems than under tightening, as you are putting a greater load on the bearings.
Once the belt is at a correct tension you can tighten the 2 alternator securing bolts.
The engine securing bolt
And the bottom alternator securing bolt
Your alternator belt is now adjusted. It may be worth starting the engine at this point and checking that the new belt doesn't squeal when you put the heater on full along with the lights. Now to put the car back together. Don't forget to put the plastic cover back in the wheel arch.
Put the wheel back on, but before putting the rest of the car back together, it might be worth checking that the alternator is charging OK. I checked the battery voltage and it was 12.48 Volts. Started the car and.....It works, 14.18 Volts.
Usual stuff applies. Although I have tried to think of everything you need, I can't be held responsible if you try this yourself and things go wrong.