I was surprised your step 1 wasn't "build a working (but ugly) prototype out of pre-existing breakout boards and components."

I might have a pencil sketch or two, but I wouldn't be doing anything in CAD until I knew my idea was feasible and had some grasp of exactly how it might fit into a finished design.

What stages did you go through for the conceptualisation and prototyping of the idea? As someone in the hardware business, I know the circuitry involved is common and simple enough that you probably had little difficulty. But I'd still love to hear how you discovered what you needed where to source it.

Good effort anyway, we need more guides like this on the web!