Flash Download Tools

Espressif has an official GUI tool for Windows.
It has a lot of options and can be used for the ESP8266, ESP8285 and ESP32.
You can find it on Espressif’s download page here: https://www.espressif.com/en/support/download/other-tools
(if the link changed, just search for esp flash download tool)

Flashing the firmware bin file

You can find precompiled .bin files on the release page. Be sure to download the latest version. The 1 MB file should be good for most devices. If you have a NodeMCU with an ESP-12 you can also use the 4MB file. But all in all, it shouldn’t matter that much.
Use one of the following software to flash your ESP8266 with the .bin file.
If you’re beginner, have a look at Node2Deauther.