Answer by
LeheckaG (1826)
To cut flagstone:
WEAR GLOVES & SAFETY-GLASSES,
Measure & mark where-to-cut,
Tape over where-to-cut on both bottom & top sides with "duct-tape" to lessen chipping/cracking,
Use a concrete/masonry-saw (or a circular-saw)
with a metal-reinforced-ceramic/fiber concrete/masonry-cutting blade.
Prevent cracking:
Support the flag-stone on both-sides of the cut & on the ends.