You can do this many different ways.
1)Score=scratch the glass then snap it.
2)Score the glass at hand then wipe some spit or water over it and then snap it.
2) Score the glass then take a hot molten blob of glass and apply it to the score and it will snap on it's own.
3)Use a diamond saw
That is just a few different ways. I am speaking about tubing primarily but this all applies to flat glass as well.