I am trying to write user subroutine but unable to start. I would like to learn it in the way to create a subroutine from scratch. Based on the formulation that we have, how to create a subroutine and which subroutine?

I was trying to search for the relevant sources, books, ebooks, websites, tutorial videos, etc. But haven't found any solid source. So thought of putting it in discussion to collect sources from different peoples from different fields. Also, this would help researchers like us in future too.

Please share the sources for learning the basics of Abaqus subroutine scripting.