Measure the length of the mantel starting at the last stone piece installed. Determine how long the cut needs to be in order to fit snugly around the mantel.
Measure up from the last stone piece installed to determine how high on the stone you will need to make the cut. Transfer all of the measurements to the stone with a grease pencil and a straight edge.
Use a diamond blade saw to cut through the stone at the measurements you marked. Use mason nippers to cut the stone if the cut is too small for the blade.
Install the stone according to manufacturer’s directions.