Practice for complex electronic circuits
You begin by starting the process, period.  This is not a linear process.  The important thing is to start somewhere.

If you're interested in online video courses, Google for them.  If you're interested in dead tree books, Google for them (as always, "The Art of Electronics" is a must have.)

If, for example, you're interested in audio amplifier circuits, Amazon will give you a number of selections at various levels of sophistication.  You'll also find a wide variety of DIY forums (of varying quality).

This site is for specific conceptual questions so this is not so much an answer as an encouragement:  get started reading and building.

When you encounter something specific you don't understand and can't seem to get past, come back with a specific question.  And, don't worry if it doesn't make sense immediately.  Often, after a period of time, what didn't make sense at all seems crystal clear.

That's what I mean by this being a non-linear process.  You'll read things that don't quite make sense but, when you review these later, after more reading and practice and letting your mind "connect the dots", they make perfect sense.