mark the glass that u wanna cut using a marker.then draw a line on the glass u intend to cut using a cloth dipped in kerosene and cut the glass using a glass cutter and a straightedge.
I would only recommend cutting your own glass if you have salvage or second-hand glass, the guy that sells you should cut it at no charge, plus they have giant wet-sanders to smooth the edges.
Like beblondie said, measure carefully twice and then score the glass using a metal straightedge. Then move the glass to the edge of your work surface so that the scored line is right over the edge of your table. Hold the glass flat against the table with one hand and grab the piece to be removed in the other. Apply a quick downward force on the cantilevered piece and it will pop right off. With large pieces, get a friend to help or clamp down the glass to the table top. I use a Dremel tool with a carbide disc to dull the edges (wear eye protection AND a dust mask).
Be sure to dip your glass cutter in a light oil or use one with a built-in oil reservoir.
I greatly thank you all for the input. I tackled a beat up 10 gallon and only made 1 successful cut out of 15. Then realized my work surface was warped, no damn wonder.