Now, insert micro SD card to your computer, then open Etcher. Using the "Select image" button, open the Raspbian image you extracted from the downloaded archive. Its name should be like "2019-07-10-raspbian-buster-full.img". Then, use the "Select drive" button to choose the micro SD card you inserted. Because Etcher is going to erase everything on the micro SD card, double-check you have no files remaining on it. Finally, press "Flash!" button to write operating system files to the micro SD card.