There's a separate forum for add-ons on these forums, I expect it's mainly for users but there may be some discussion for add-on developers:
https://forums.elderscrollsonline.com/en/categories/add-ons-and-ui-mods
You could try the esoui guide.here: https://wiki.esoui.com/Writing_your_first_addon
Don't know if there's better options out there though.
Note, when you are testing your own AddOn, you can't make changes while you are in-game and have a character loaded, even just having your AddOn code open in a text editor can prevent ESO from loading it properly.