Hey, I am one of the OS maintainers. Step by step instructions are difficult to provide as all boards are different and will require slightly different tweaks. The process is basically what is outlined in the documentation and in the video you mention.
Spend some time getting to know the hardware you want to support, in particular the booting details
Make sure you have can fluently work with Yocto Project
Choose a suitable BSP for your board at the recommended Yocto Project version (currently Kirkstone)
Prepare the device repository following the documentation
Integrate the kernel and bootloader customizations
Build it
Boot the image and debug
Add support for peripherals as WiFi drivers etc
Pass the OS automation test suite, manually or automatically
BTW, I briefly looked at your site. Nice use-case - lower costs, lower carbon emissions.
I think your question has been answered. So I am going to close this ticket. But if you have follow-up questions, please feel free to reply and the ticket will automatically re-open.