Hi James Eyre,
As it happens I found myself scratching around for similar kinds of resources just the other day. While I didn't come across any how to guides as such, I'd recommend taking a looking that the samples produced by the ArcGIS Monitor team over at https://arcgismonitor.maps.arcgis.com/home/gallery.html
Once you read through a few examples a consistent pattern starts to emerge, whereby the extension is written for node.JS and the index.js exports an execute function that returns a deferred promise to the collector, and an inputs object is exported to define the parameters of execution. In many cases the business logic of the extension is wrapping a powershell script or a compiled binary, but on the javscript side they all follow a pretty similar kind of pattern. In particular I found the File Read Write extension a good one to take a look as it demonstrates the general concept without getting too weighed down in additional complexity.
I haven't gotten around to writing any of my own extensions for Monitor yet, but I see a lot of potential in this space and I'd love to see some more doco and/or hear from anyone that's done this before.
Anyway, hope that at least helps with getting started!
