First, get a glass cutter, something like this:
A steady base can be useful, to cut on top, and a large ruller, to maintain the line straight!
Then get your line marked, press hard on the edge, and just follow the ruller!
In the end, a small tap, all along the cut line, to break it lose!
With this specific cutter, is easy to cut glass up to 5mm, more than that, and the edges might not get cut properly!
Here you can see, how it was hard to get the edges cut, this glass is 6mm, from my turtle aquarium lid!
