[Dessesaf]

Like anything else, gradually.

You really don't have to understand any deep wizardry to get started (or, for the most part, even to finish). For the most part, you just look at a specification, and implement what it says. It requires some code architecting skill to not make a mess, but there are common patterns and it becomes a lot easier once you've built one or two emulators.

And you almost never need to understand electronics. You're only emulating behavior. When someone discovered bugs in the behavior of the original hardware, you usually just need to special-case it in your emulator. It might help to know some electronics to understand how those behaviors came to be, but that's more so of historical interest than actually practical.

There are certain unique challenges, but it's nothing too difficult. When there's an issue, you're usually debugging three things at once: Your understanding of the hardware, your implementation of the emulator, and the game you're emulating. It can be hard to pin down the exact problem. But here, I encourage you to just hack something together. It's not clean, but all emulators are full of special cases that try to somehow get popular games working. If a couple unclean hacks mean you can get a game working, just do so. You don't really need to exactly implement the original hardware's behavior. Just get the game working.