For this guide, we will use the CH340G as it is the most common in current clones.
+---------------+ | | | USB CONN | | (U2) | +---------------+ | | v +---------------+ | | | VOLTAGE REG | | (U3) 3.3V | +---------------+ | | v +---------------+ | | | ATmega328P | | (U1) | +---------------+ | | v +---------------+ | | | POWER LED | | (LED1) | +---------------+ | | v +---------------+ | | | PIN HEADERS | | (J1-J14) | +---------------+ | | v +---------------+ | | | PIN 13 LED | | (LED2) | +---------------+ arduino nano eagle schematic