Running pliers. The name of the special pliers that have a line on them you can line up with the score you make on the glass, then squeeze gently, and the pressure "runs the score." Also grozier pliers, I use for yanking off that 1/8" sometimes.
Definitely practice, esp. if you get a stained glass person to show you how to cut glass. Glass is temperamental, for example, when it gets cold, you can forget the score line running clean; it has a mind of it's own! Art glass, the stuff they (we

)use is typically thicker, different textures, so the pressure it takes to cut that kind of glass is a little more than glass used in framing.
Slow even pressure, like Baer said, listen for that sound to be consistent. It it crunches, or flakes off, you are pressing too hard!
I like Jerome's sugg. of drawing the line to follow underneath the glass. I pull my cutter towards me, and watch the wheel. Others push. Up to you. When you are lining up your cutter wheel with your measured line, make sure your wheel is on the line, and put your straight edge flush against the cutter. This will make so much more sense when you have an oiled cutter in your hand.
Baer, none taken, not Georgian. I am from M- i- crooked letter-crooked letter -i, crooked letter- crooked letter i, hump back-hump back i!
Welcome to the G, sandym, where in MS are you?