Find a pure crystal glass. In order to break this glass using your voice, you have to find out what its natural frequency and pitch is. You can do this by tapping it with a fork or piece of metal.
Match the exact pitch of the glass with your own. It may take some time, and much practice, because the pitch must be precisely matched. The science behind breaking the glass is advanced, but it comes down to finding the natural frequency of an object, and add enough force and power to that frequency and it can be destroyed.
Begin singing in the same pitch. When trying to break a glass with your voice, you may have to hold the note for quite some time.
Vary your pitch if it doesn't work on the first try. Many people who are able to break glass with their voice are highly trained individuals. It takes a lot of skill to get a note accurately.
Add power to your voice, in the precise pitch, to break the glass with your voice if you hear any cracking or breaking. This should help with your endeavor.