Well I suppose everyone learns very differently but for me it was mostly trial and error …but you sure need the passion to be oke with that.
I’m trying to remember how I went about when I started flight simming 21 years ago, aged 14. I thought I knew quite a lot back then, using my keyboard arrows, taking off the default 737 in FS2004. …I didn’t know anything though, but I only learned that later went I finally hooked up with PMDG. Haha.
I’d recommend to follow the rule of thumb in aviation: Aviate, Navigate, Communicate. So take off with your Cessna 172 and fly those patterns around the field, do you go arounds, “feel” the airplane. I’d say a lot of aviation comes down to sophisticated gut feeling and well trained instinct. Meaning that you fly it from the seat of your pants, even that 777, but it adds up with all the knowledge in your head as you go along. That does take time though. Trial and error.
Anyway, once you feel your plane, make it go somewhere. Make a one hour vor-to-vor route, operate your navigation radio’s, learn how to use your HSI (to make it follow a course to a beacon), pass your beacon, quickly have the next one ready and repeat and repeat. I’ll promise you this is a quick way to understand the fundamentals of navigation. To take your plane somewhere and see that practice follows theory. Keep doing this for a while and you’ll get comfortable with it. Screw GPS-navigation, that doesn’t learn you anything. Anyone can flip a few buttons and follow a magenta line: that doesn’t mean you understand anything about it.
After all that, learn and understand about procedures with ATC, patterns and so on. Once you got the first two, the last one will make a lot more sense as you understand why and how they are designed as they are, to what purpose.
…Finally. Moving over to sophisticated jets eventually. Well, back at PMDG, I just started to read the manual and slowly understanding what makes a jumbo jet a jumbo jet. It’s not so much one integrated machine as you might see 'm at first glance. They’re more of a collection of systems and machinaries coming together on that flight deck. Just learn 'm one by one and you’ll eventually get the cohesion between all 'm.
After you get those basics: go to youtube and see very passionate people talk about their planes and the procedures they find themselves in. You can’t ever grow bored from that once you get that bug.
Have a great ride! This hobby sure never bored me these past 21 years.