Study http://github.com/volumio/volumio3-os to start with.
The community development, aiming at porting to different platforms, is not very active anymore.
But in the repo you will find some good recipe examples for armbian-based os files, not very difficult, look at the rkbox versions and the platform files it refers to. It pretty much describes the basics.
Note: when you manage to build an image for these boards, they will not have myVolumio, lacking all premium features. Building a version which includes it, is restricted to Volumio developers (policy, non-negotiable).