Glass grinders are very handy tools. They mainly consists of a spinning diamond coated cylinder (the bit) extending above a grid surface that can rapidly and accurately grind glass to the desired shape.
Don't purposely cut glass large with the intention of grinding. Ideally no grinding would have to be done. It should be used only for minor adjustments or for grinding tough-to-cut inside curves.
The diamond bit must be kept wet in order to reduce wear on the diamond and prevent glass dust from developing and being inhaled. There are several grits available. "Fine" grinds slower but leaves fewer chips in the glass. "Coarse" grinds very fast but leaves larger chips. "Standard" is a central compromise.
Ensure there is water in the reservoir before starting any grinding. Empty the reservoir daily. This keeps the water from producing a smell, and allows you to clear the glass residue from around the grinding bit.