I am trying to install a small amout of decorative half round cement siding. What fasteners should be used, screws ? Type ? Nails etc. How to prevent cracking? What should the under surface be? Application in general

Usually, some kind of nail gun is used for cement siding. I'd have a look at the manufacturer's web site, or any pamphlets that came with the material. You should be able to find specific directions there regarding size and spacing.

You may wish to spend a few extra dollars and buy stainless steel fasteners. It avoids the rust stains on siding that can occur after a few years.