When cutting fiber cement siding with circular saw use a blade that has only four or six teeth.
How to cut hardie lap siding.
This is the same way of cutting the drywall.
In fact you could use a utility knife or a carbide tipped scoring equipment in order to spot the line.
The easiest method to cut the hardie board is that you spot the surface and break the sheet along the marked line.
To keep dust to a minimum use a circular saw that s equipped with either a dust collection bag or electric fan that blows dust into a bucket.
The sheet should be removed immediately after each plank is installed.