I recently wrote the following to a mechanical engineering student who was wondering how to develop practical electrical engineering skills, specifically in terms of electronic component selection, circuit design, and consumer product polish. While the following is an N=1 anecdote I do believe that this approach can work for many people and many fields as well.
This describes me in college. I was a mechanical engineering student; I loved electronics and programming but was disappointed that I didn't really get my circuits 1 class.
I decided to dive into mastering one type of electronics skill. So instead of hopping around Arduinos and microcontrollers and DSP and robotics and control systems and other topics, I did just one thing: I started building guitar effects pedals (I'm a musician, so it was fun and useful for me).
At first I just found circuit diagrams online, got all the required components, and figured out how to board it up on various media (bread boards, perf boards, eventually PCB). I didn't really understand why each component was used, I just blindly followed the diagrams and built circuits. Even without understanding what everything did precisely, I still had to debug broken circuits and that was a very helpful skill to learn, because you learn how to use your tools better.
Anyway, because I stayed on guitar pedals and audio circuits, even though I was only emulating and not inventing, I started to see common tropes. Eventually you realize, oh, this is an input filter. This is a power conditioning filter. Output filter. This is a high-gain boost. This is a rectifier. Etc.
Eventually you start seeing complex circuits as many functional sub-circuits. And once you get to that point, the selection of individual components becomes clearer.
For instance, in a power rectifier you use one type of diode, and it typically doesn't matter precisely which model you use. But rectifiers are used in certain audio and signal processing applications as well, in which case the selection of diode really does matter. You're looking for diodes with certain properties; for instance, in certain distortion effects you send an AC audio signal through a diode in order to make the wave asymmetrical. Now if you used a perfect diode, the bottom half of the signal would be cut off flat. But you don't really want that (it introduces harsh harmonics) in an audio signal, so you hunt around for a diode that has a certain breakdown voltage in the reverse direction, so that the signal doesn't cut off sharply but instead is heavily muted on one side.
And of course, that's just one example of component selection, but I share that anecdote because that's how you ultimately come to see every component in a circuit. This capacitor is used as a power conditioner, so let's pick a big heavy electrolytic, polarized capacitor. That other capacitor is used as a signal conditioner, though, so it needs a certain capacitance and some other properties as well, so we have to choose a certain type of ceramic disk capacitor.
And of course, as I went on with this, not only did I learn how the circuits were designed, I learned how to improvise (change certain values of components here and there to get other qualities from the circuit), and ultimately to design circuits from scratch.
At the same time, I also worked a ton on the polish. I started building beautiful enclosures for the circuits in the shop, but I also started designing and printing PCB boards at home. You can design a PCB layout in any number of programs, and you can get copper-clad board and PCB etchant easily. But here's the fun part: you can print the circuit layout on photo paper with toner, and iron it on to the copper board. This leaves the copper covered in toner where you'd like the traces to be. You immerse the board in etchant and wait, and the unprotected copper dissolves away. Then you grab a drill press (or, in my case, a Dremel and a clamp) and a 1/16" bit (tiny!) and drill those 50-100 holes and solder everything in.
So, in about 18-24 months, I went from no practical knowledge of circuits, to being able to design and build consumer-grade, high quality circuits entirely from home! And I only needed a shop for drilling into the metal enclosures (guitar pedals get stomped on a lot, so you need a drill press when working on a metal case).
I don't think I would have gotten to that point had I not focused in on one area. Staying on one topic let me really dig into the specifics and polish, which turns out to be the difference between an amateur and a professional. And don't worry about having narrow skills; once I was comfortable with circuits in general it was trivial to move to other fields, and I became an all-around electronics expert.