Determining the correct grit can be challenging, as it depends on how deep the scratch is, but start with a fine grit, and go towards coarse until the scratch is reduced, rather than starting with a grit that is too coarse, and spending more time on the job. Sanding glass removes material and the coarse grits remove more.
The fewer steps necessary to get the job done, the better. Otherwise, there is a risk of optical refraction. The polishing step takes longer than the sanding steps.
When sanding and polishing, keep the polisher flat to surface, this will give a better and smoother finish.
Clean in between the steps to avoid dust from previous steps causing any scratches.
Throughout the entire process, do not touch the glass with bare fingers, as transferred grease will affect the result and in worst case fingerprints will be visible. Use gloves!
Always use a dust protection mask. Please check local legislation about concentrations known to be hazardous.
Take sufficient time over each sanding step, it is all about giving each step the time it needs.