Try this link from Priddy Chimney Sweeps…
Essentially, chimney flashing functions as weather stripping for a chimney. It is comprised of three major parts: #1: Step flashing. #2: Counter flashing/Cap flashing. #3:Base flashing.
#1: Step flashing consists of a L-shaped piece of metal, or other suitable/durable material, that lies underneath roofing shingles and along the brick of your masonry chimney. Step flashing should move independently of the roof deck or chimney. Step flashing should not be fastened to both the roof deck and chimney.
#2: Counter flashing is also an L-shaped metal, or other suitable/durable material, designed to cover the step flashing below. The top is embedded into a groove or cut in masonry or imbedded the bed/head mortar joint or brickwork.
#3:Base/Cap flashing is installed at the base and head wall of the chimney.
You will read different terms and methods.