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Good ways to start:
- Learn to use the hook system.
- In case you don't know which hook to use, browse the PHP files and look for the hook locations. This is kind of fun actually.
- https://members.vbulletin.com/api/
- Download (small) existing mods and look through the files and the installation .xml file so you understand how to make plugins in the future.
- Make small mods first, maybe something to alter the postbit template, etc.

Well, I read all articles here: https://vborg.vbsupport.ru/forumdisplay.php?f=184

I learned by doing. I started by modding my own site. I always wanted something slightly different from the modifications I got here, and so I started by learning to modify modifications. As I got more confident, I started making bigger and bigger modifications to them and eventually started writing my own mods for my site. Having a test forum is essential! :)