Cutting glass must be one of the more difficult chores that a craftsperson has to do. This is usually done when the size of the glass is custom-fit, or when the artist is just trying to save resources by recycling the glass that he/she already has in stock.
If you're someone who doesn't want to spend a lot of time figuring out how to class your glass, then having someone professional do it is the way to go. Otherwise, I am telling you that it can be done while you're at home. It just needs a little practice and patience on your part. It is not a perfect art, but it can most definitely be done.
First, make sure that the surface of your glass is clean and free of silica. This will ensure that your cut will become more accurate. Prepare your glass cutter and some light oil, and grasp your cutter in a typical cutting position, with the tip of the blad just touching the glass. Dip your cutter in this oil, to make the score smoother. You may want to use a spare piece of glass to test your pressure. There should not be much gritty sound, which means you're putting too much pressure.
To ensure that you will be cutting in a straight line, use a ruler or anything with a thick straight edge to put against your cutter. Smoothly cut the surface, leaving a scratch-like mark on it. Grasp both sides of the glass and simply snap it into two, taking care to move your wrists only. Then use a sand paper to remove the rough edges.
I found this instructional video about the same topic. I hope that it gives you a clearer understanding of things. In this video, the glass is specifically cut for custom picture frames.
