I find sketching schematics for hardware is a lot more natural than sketching UML, and they're super helpful for making sense of a rat's nest of wires. I can't recommend them enough! I'm a big fan of using paper, it's quite efficient and easy to edit. I only use a design app to present the schematic.

Most electronics books will teach you the basics, but this StackExchange answer is a good summary too: https://electronics.stackexchange.com/questions/28251/rules-...