Jack up the passenger side of the car and remove the wheel
Remove the splash guard.
Remove both belts if a L67, or the Acc belt if not supercharged.
You will need to remove the access plate between the engine and the transmission. You will then see the flex plate. Put a strong punch threw a hole and lock the flex plate in place. You will need to remove the starter to have clear access to the flex plate.
Spray some penetrating oil on the nut. If this does not loosen the bolt, use some heat.
You will need a 15/16 hardened 6 point socket and a large beaker bar. This picture shows a engine outside the car but can be done while in the car. You will need a long hardened extension and a jack stand to hold the extension in place. your going to have to really push. Be careful that if the socket slips, or something brakes, you do not do a face plant or smash your hand.
Back the bolt out about a inch.
The balancer puller I picked up didn't have the correct bolt size. You will need fine thread 1/4" bolt, and 4" long
Put your puller in place and thread in the 1/4" bolts only about a 1/4" in. And place the flat adapter on the end of the pusher.
Now start turning the threaded bar to pull the balancer off. This will come off rather easy. It'* far more work betting the bolt out. Watch the bolt, if you run out of pulling room, back it off and move the bolt out some more.
Once the balancer is loose, remove the puller and then remove the bolt.
Then remove the balancer.
Make sure the end of the crank shaft is clean.
Now if you need to, replace the crank position sensor. http://www.gmforum.com/t279066/
Place the new balancer in place and line up the key.
Put a new bolt in place. You must use a new bolt as this is a torque to yield bolt and will stretch.
Tighten the bolt up.
Now with the fly wheel still locked in place, torque to 111 foot pounds, once you get that torque, rotate the bolt another 76 degrees. This is the stretch.
Put your splash guard back.
Put your wheel back on.
Now start the engine and check to make sure your belt is tracking correctly.