I'm unsure when an idea of something that might work becomes an invention. I'd have thought one needed a working prototype first.
My guess is that whilst it depend on the idea, and what it involves, it has to be to do with getting the finance, (getting backers if you can't pay for yourself) and then discussing with local firms to get the bits made for you to assemble.
Unsure about patenting but I suspect the patent office could tell you the details. What pictures/writing you need to supply. But the idea is supposed to be original, and not already publically known.