how do you make a mould? is it easy?
A short answer is.......No.
Of course it depends on how accurate you want the mold to be and how much equipment you have and how much your willing to spend on some.
The typical round ball mold is formed with a "cherry", which is just another name for a spherical mill cutter.
Having the equipment to power this cutter, and to position it in the right place relative to the blocks which are being machined is helpful.
Then there is the little thing called "shrink factor" which is the amount of oversize the cutter has to be to produce a ball of a desired size.
In other words, the cavity has to be larger than the cooled casting is going to be.
This is dependent on the "Alpha" (coefficient of thermal expansion) of the material being cast and the size of the finished casting.
Take my word for it, it's just a lot easier to buy finished mold blocks or if a special size is wanted, to pay someone who has the knowledge and equipment to build what you want.