[sammyg2]

Two common ways to do it:

1) mechanical i.e. sand blast or diamond grinder.
Tough to get it even and to keep from weakening the glass. Borders can wash out as masking gets eaten away.

2) chemical etching.
Usually involves diluted hydrofluoric acid, (aka HF or alien acid). Very effective but potentially very dangerous stuff. It does not lose it strength when diluted, it just takes longer to kill flesh. keep it off hands and out of eyes.