Den,
You just need absolute confidence.

Glass has an uncanny knack for spotting a nervous or inexperienced amateur attempting to cut it, and invariably won't comply, breaking at the most inopportune point of the job, and given any chance will result in a plentiful flow of blood.

However glass will always snap cleanly and accurately out of respect for the skill of a time-served professional, who has a warehouse full of glass, unlike the amateur who only has that bit he just bought and carried home with such care.

I hope this helps?