Using the device:
- Fire up your favourite flashing program, like Arduino IDE or Tasmota-Pyflasher
- Plug the UART cable/PCB to the device
- Plug an adapter to the device's Gnd-Tx-Rx-Vcc pins, if needed (many devices don't)
- Plug the GPIO 0 wire to the device and target/adapter, if needed (some have a button for GPIO 0)
- Connect the rest of pins to target, at least test fitting before connecting power
- When connected OK, hold it and plug in USB (for the next ones you already know how it works so can keep it connected)
- Switch ESP to flashing mode: press down power button (LED should go off), press down GPIO zero button, release power button (LED should shine again), release GPIO zero button.
- Start flashing
- After flashing, the device can be switched to normal mode by pressing the power button for a second or so to empty capacitors. Most WiFi switches don't serial output though, these you'd generally unplug, assemble and plug to wall.