A Mold
To even get started you need a mold. Making a mold is a time consuming process but not difficult if you have a large pile of scrap wood and the odd nail. A set of rules and plans will come in handy too.
I managed to avoid this step of the process courtesy of borrowing one from Martin. If you are not so lucky read on.
[TODO a pile of sketches and an explanation of how to build the mold]