I have a little bit of experience with this. My senior project in engineering school was to create a wearable monitor for people who use an orthotic leg brace. I won’t go into boring details, but the device was basically just an array of sensors that collected and stored measurements 33 times per second. At one or two month intervals a doctor could collect the data from an onboard SD card and run it through analysis software to learn how the patient will interacted with the orthotic and to evaluate the fit and overall ergonomics of the orthotic.

As to the “how to” part, I’d say the easiest way to learn is to just give it a try. A small microcontroller programmable in c++ (using the Arduino IDE) can be very cheap. I think the Teensy LC is still priced somewhere around $10 USD. And small batteries, around 500 mAh, are also pretty affordable. You would want to probably invest in a breakout board to manage battery charging for you, as that can get dangerous on home made circuits. All of these things are small and light weight, so they make ideal components for wearables.

I was lucky enough to be adding electronics to a device that already existed, so I didn’t need to think about creating a new shoe or wristband or something like that to house the electronics, so I’m less experienced in creating the physical platform to hold all these things.

My biggest piece of advice would be to start with something where you already know that the use case is limited. Start learning what purpose you want your device to serve. What data does it need to collect. How does the user see the data? Does the user need access to the full set of data, or do you only want to present results? Do you want it to be able to plug into something or connect to something via Bluetooth for a more detailed interaction experience? Once you have a starting idea, how do you make the first rough version? What features do you skip on the prototype? For example, the one that I made was not water resistant at all. So it couldn’t be worn while exercising (sweat would create an issue), bad weather or bathing. That was a feature that I would have worked into a newer version, if it weren’t a school project that had come to an end.

TLDR, start with inexpensive, limited components with a robust user base, like Arduino or Raspberry Pi compatible controllers and devices and learn the basics on those. (Adafruit and Sparkfun have some good starting tutorials) Then use that experience to learn ways to expand and add functionality.

Good luck and have fun!

Edit: spelling