Honestly, it's just a matter of knowing this list:

• CPU
• RAM
• motherboard
• GPU
• hard drive
• case
• power supply

And roughly how they should fit together.

But every time I build a PC I have to figure out what the latest versions of these parts are, make sure they're compatible, and when I get the parts they might have some unique form factor I have to figure out on the fly. Just going to PC Part Picker and picking out each part is 90% of the way there. After that it's just a matter of getting them, sticking them together, crossing your fingers that it powers on, and installing an OS. If/when it doesn't power on, THAT'S when you start learning...

But I would say building a PC is not a fraction as difficult as say, knowing how to work on a car.