Start by deciding its purpose. The form will follow the purpose. If you know electrical engineering, you won't make as many mistakes as I have. If you know mechanical engineering, you won't take as long to design the components as it took me. If you're a competent programmer, you won't be as slow integrating the hardware into a cohesive whole as I am. If you've read The Design of Everyday Things, you're ahead of the curve in terms of making it accessible.