Cutting flagstone isn’t as difficult as it looks, depending on what kind of cut you need. For outdoor patios, landscaping and other common uses, you’ll frequently need an approximate cut rather than an exact one. In that case, it’s often a matter of creating a controlled break in the stone rather than a precision cut. When you do you need exact cuts–say, for a set of steps–talk to your local home improvement store about renting a masonry saw.
Hammer and Chisel - For cuts that have to be approximately (but not precisely) straight in relatively thin flagstone, you can break it along a scored line. Set the stone on a solid setting. Mark your line on the surface in chalk. Use a hammer and heavy iron chisel to chisel the line into the stone. Once a groove has been cut, continue tapping at it with the chisel and hammer, moving it back and forth along the length of the line. Don't hit it hard; the point is to chip at it gradually until the stone under the line weakens enough to break.