Scribe the Glass
Mark your cut lines on the glass panel. Use a T-square and a glasscutter with carbide wheel (Image 1) to scribe the glass.
Run the cutter across the width of the glass. Put the tapping ball under the scribe (Image 2), apply a bit of pressure and the glass will snap off.
Do the same thing for your vertical cut. It's okay if it breaks off in several pieces. If there is a small piece left, use the teeth on the end of the cutter to break it off.
Smooth the entire length with a rubbing stone (Image 3).