The gist of it, as mentioned in [4], is that you need to have a web server that implements checkStatus and saveConfig PUTs, and PUT and DELETE for saveBoard.
[1] https://github.com/apankrat/nullboard-agent
[2] https://github.com/luismedel/nbagent
[3] https://github.com/OfryL/nullboard-nodejs-agent
[4] https://nullboard.io/backups