...is to pull it apart.
I've reverse-engineered various things, including power supplies, interface cards, as well as ROM-based programs and even programmable logic devices, and even made my own shortform datasheets of ICs I cannot find a datasheet for.
This helped me understand how things worked, and what to do if they don't, and even learn how to build my own, such as an infrared receiver module pin identifier from my completely original design.
I started with game console equipment back in 2001, to help fill in the gaps from information found online.
When my local junk shop Trendy Trash opened back in 2003, I bought various items cheap from this shop and discovered how they worked, and even had a chance to try fixing them.
There are some things I cannot find a circuit of anywhere (or is very hard to do so); so I made my own circuits after some reverse engineering.