Start researching what you want/can do; look at different hardware options (physical size will matter); and get a prototype working on your bench with an Arduino and breakout boards for the various components you may want to use.
When that's working reliably (just replace that relay with an LED while testing) it's time to look into using barebones processors (so without bootloader, without USB connectivity, without built-in regulator, and all the other goodies and protection an Arduino development board comes with). You may consider an ATtiny for less size. Build it on a solderless breadboard. When all works, try replacing the LED with the actual relay, make sure it still works (power supply issues may pop up).
We're a few months into the project by now.
Next step: power supply. Automotive power (so also motorcycles) are notoriously noisy and need very good filtering, decoupling, overvoltage and voltage reversal protections.
Then, time to look how to build everything onto your motorbike. Find a suitable container (water/weather proof etc).
Now you know how to program the chips, have your circuit working and tested, software (mostly) finished, and the actual relay working reliably, it's time to get out the soldering iron, and solder everything firmly onto a piece of perfboard, sized to fit in your container.
Then test again, and when confident it all works reliably build it onto your bike. Work regularly on it and you should be able to have it done by next summer's biking season.
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