All the comments here are very true. But ultimately, they lack the main point, which is unless you’re in a strict training regime, we all learned IFR operations by trial and error.
The original FSX came with tutorials that taught the very basics of radio navigation and IFR flight, but introduced really subtle concepts like approach plates. Naturally, if you’re interested in the subject, you end up looking around for more approach plates, which inevitably introduce you to new concepts that you look into, thus introducing you to new concepts, and so on.
The thing with IFR flight, when most of us weren’t using this as a training tool, is continuous exposure to new things and learning about how tools apply in certain situations, and how they apply in new and exciting cases.
I would say, from my first exposure to learning how to fly an IFR flight independently to when I would say I was fairly proficient and knowledgeable was…roughly 6 years (freshman year high school until one day in college I decided to become a VATSIM controller for fun).