Hey Allonsy,
So I have bookmarked several resources you could use to get started:
The following resource has a free course, Basics Concepts of Synthesis:
https://learningmodular.com/
There is also a book that complements the course, it really helps broaden your perspective by looking at the history and context of modular:
https://bjooks.com/products/patch-tweak ... -synthesis
They have a nice patch editor that would help you study and break down patches:
http://editor.patchandtweak.com
The Cherry Audio YouTube channel has plenty of things to get you started, I recommend spending dedicated time on that. Create playlists based on the tutorials and work away:
https://www.youtube.com/c/CherryAudiovst
There are also a few magazine issues from magazines such as Computer Music, Electronic Musician, Future Music, and Music Tech that could also widen your perspective. A Google search will help you find what you're looking for in terms of the specific issues. Key terms: modular.
Lastly, I would take simple patches and study them. Break them down, take notes, and try to recreate them from memory. Commit the simple steps to memory. You'll find a few core archetypes emerging from this process. You could save these as cabinets/presets to help build your library.
In terms of self-study, this would help you on your way. You could always find a teacher/mentor as well but I am assuming this is an autodidact project.