the location of the belt tensioner is in a tight spot. anyone who has replaced one and has normal size hands knows what I'm talking about. I'll try and make you a step by step procedure below. For visual reference, please view my BIO to gain access to my Saturn help site. In the serpentine belt folder is a picture of the tensioner and the two bolts that hold it in place, so you get an idea of where they are in relation to the tensioner unit.
1. Raise and support the vehicle with Jack stands on both sides
2. Remove the front passanger side tire (19mm lug nuts)
3. In the wheel well, remove the splash shielding to gain access to the motor.
4. Place a decent size peice of wood on your jack, and slide it under the oil pan, raise the jack so it firmly presses on the oil pan.
5. Under the hood, remove the motor mount on the left side of the motor, the mount is held on by Three 15mm nuts with studs on the motor itself, and Two 15mm nuts holding the mount to the body. remove the mount completely.
6. slowly lower the jack supporting the engine until the engine sits all the way down.
7. from inside the wheel well, place a 14mm wrench on the belt tensioner pully bolt. swing the tensioner to remove tension on the belt, remove the belt from the tensioner or main crank pully.
8. You should have access to the lower bolt on the tensioner. the bolt is 10mm in head size. i recommend using a 1/4" ratchet. Remove the bolt.
9. raise the jack as far as you can, making sure the vehicle stays on the jackstands!!! this should allow you to reach down and beable to remove the top bolt on the tensioner. the motor will come up on an angle.
10. once the bolt is out, the tensioner should just fall out of the motor. reinstall the tensioner with the top bolt first, lower the engine, reinstall lower bolt, reinstall belt, reinstall the upper motor mount, and raise the engine to level out with the upper mount.
The job sounds complicated if you are not mechanically inclined, but its accually simple. you will not break anything by moving the motor. the job should take a average person about 30-45 minuites to complete.