The link describes it nicely.
Ensure that you do it by hand (no power tools) and that you use a solid sandpaper block to keep the sandpaper flat on the crystal.
I use 400 grit to take any nasties off, 1200 for smoothing and then duraglit padding (same stuff as brasso / polywatch) to finalise.
Also make sure that you use masking tape on any area of the watch (bezel etc) that you wouldn't want accidently sanding.
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