belt replacement

Can anyone explain how to replace serpentine belt on a 05 Ion, Thanks

i usually do it from under the car, but it is possible to do it from the top. from the bottom, you have to take the right wheel off. then take the wheel liner off. this consists of about 8 pushpins (approx.) a seven mm screw and a ten mm screw that attaches the air intake to the frame. then you'll need something (ratchet or belt tool) with a 3/8 head to relieve the tensioner.
from the top, youl'll have to jack the car up, support the engine under the oil pan, take the air filter housing out, remove the upper mount, then use the 3/8 tool of choice to relieve tension. it's not too hard, but easier with a lift.

Does anyone know which is the best belt to use?

Any of the name brands will do. It's a very simple serpentine belt setup on the 2.2L. The OEM belt should last a very long time.
http://media.gm.com/us/powertrain/en/product_services/2007/Photo%20Library/Ecotec/L61/07%202.2L%20I4%20L61%20ION%20LoR.jpg

Joel