How to break a mirror into pieces for a mosaic. This technique covers the basics, you can practice the technique to expand the range of shapes you can create for making mosaic pieces.

Step 1: Score the Glass Mirror

Align the straightedge and press firmly on it, or clamp it firmly. Then press firmly and evenly with the glass cutter to draw a line across the face of the glass.

Draw only once, avoid sawing motions.
If your wheel sticks, then add a drop of cutting oil.

You can make other shapes, I chose strips for simplicity.

Step 2: Preparing to Cut

Align scored glass along top of broom handle.
Cover it carefully with cardboard so that the mirror doesn't slide off the score mark. This is the only tricky part, from here on you have to know by feel if you are on or off the score mark.

Then cover with your wood or other firm board.

Step 3: Cutting

Apply pressure evenly along the score mark,
and push/lean towards the small side, it should snap right off.

If it doesn't then check the alignment of the score mark along the broom handle. Then try again.