http://www.priorypolishes.co.uk/shop/front/index.php?act=viewProd&productId=45
should help
Do not use abrasive household cream cleaners. Whitening toothpaste is a good alternative to polishing compound.
If possible, lay the glass on a flat surface during the polishing process to provide additional support.
If your fingernail can be inserted in the scratch, it is probably too deep to be buffed out.
Use gentle pressure to avoid breaking the glass. It is also important that you do not use a polishing compound that is overly abrasive or you could end up damaging the glass further.