Can anyone help point me in the direction of some documents for building a DIY focuser and how INDI actually uses a focuser?

I can find examples of people having built their own and masses of documents on how to add the focuser to INDI, but what I'm struggling with is finding out how INDI expects to be able to talk to a custom driver and how you turn the thing once you have it up and running. Having never used an automated focuser, part of it may be me not knowing what to expect.

I have designed and 3d printed a motor mount and cogs, added a stepper motor to a Raspberry Pi GIPO (running astroberry) and have a very basic python program which tests the concept by allowing focus control with the arrow keys. If I'm going to develop it further then it would make sense to make it INDI compatible. I'm assuming INDI sends certain messages to a running driver and expects something back. I've been unable to find what it sends or how it talks to it. All my googling has shown people who have just done it.

Once you have it talking, I was expecting to have a couple of controls to click, to focus in and focus out, perhaps being able to change the speed/granularity of the focus. The best I can find of use is a video showing someone with a similar DIY unit:
www.youtube.com/watch?v=3-pNfhXXu14

They seem to have built a few presets of the position number for where they want the focuser to go to then click go. Is this the way INDI works? While you work out what the focus position is, do you just type in say 1500 and go, then 1502, then 1504 etc until you find a crisp focus.

Any pointers on where to look for DIY drivers appreciated. I'm happy with any language.

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