first step is read that tutorial, but then, If you want to develop it interactively, you can do it on https://tw.thissma.fr/ (full disclosure, it's beta)
upload your map or create one and add a layer with an image
select the automapper (</>) tab at the top
new automapper
name is the image name (without .png)
kind: DDNet, unless you want to try rules++ which is cool too.
hit Preview when you want to test it on the map (your layer must have selected one of the automappers configs)
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(full disclosure, it's beta)
