Connect the Serial Bridge ground to the supply ground.
Connect the Serial bridge to the PC and open the ESP8266 flasher.
Select the firmware in config tab
In advanced tab ,select the flash size, if your not sure about the flash size of your ESP, start from 512Kb
In operation tab select the COM port of the Serial bridge and click on flash. Once you have clicked on flash disconnect ESP RST pin from ground, then disconnect GPIO 0 from ground. The flashing will be completed in couple of seconds.