In order to use the ESP-01 Wi-Fi relay module, a programmer/programming adapter is required to program the ESP-01 module (see below). Also it is pretty easy to make your own programmer, or you can ...

A lot of people have used ESP8266 to add inexpensive WiFi connectivity to their projects, but [Oscar] decided to take it one step further and program an Arduino over WiFi with the ESP8266.

I followed [cnlohr’s] direct programming guide to write the binary using esptool. Both this method and the Arduino method are directly programming the EEPROM on the module.