I usually start out with something like 180gr to break up the top surface of the glass, then finish up with something course like 80 or 100gr to give it a good surface for your striker to bite into. Make sure you check your sandpaper regularly, because it will fill up with glass or crystal dust very quickly and won't break up the top surface very well. You can also use the conditioner if you want to help break up the top surface, then clean it up with 180gr or something similar and again finish with 100gr.
This is just my technique, some people may do something completely different but this is what I've had good results with.