If you are using a Raspberry Pi, the best approach is just to follow the guide over at the Tuya-Convert git and then skip down to Configuring Tasmota for the Brilliant Plug. This approach has been tried and tested and just keeps getting simpler. Over on YouTube digiblurDIY has a great guide following from the Tuya-Convert procedure which is great for beginners.

We need to connect to the serial programming interface of the ESP8266 chip. This is done by connecting our serial-to-USB converter TX and RX pins to the ESP8266 RX and TX pins and powering the chip with the 3.3V and GND pins.