First you need a firmware file. You can build one yourself with the Toolchain or download one:

• Espressif firmware source updated at http://bbs.espressif.com
• Electrodragon provides a custom v0.95 AT Firmware based on espressif SDK 0.93. (list of changes)
• NodeMCU - firmware for running Lua scripts, or see the forum article Build your own custom NodeMCU firmware in the cloud
• MicroPython - firmware for running Python 3 scripts. Supports several microcontrollers including the ESP8266.
• Other projects listed on the user projects section of the wiki home
• See this site for a prac.http://www.xess.com/blog/esp8266-reflash/

To flash the ESP8266, you'll need first need to wire things up. See Getting Started with ESP8266 for the basics.

Connect GPIO 0 to ground and reboot the ESP8266 to enter flashing mode.

The NodeMCU flasher is designed for use with the NodeMCU firmware, but it can also be used to flash other firmware files. Windows-only currently, but a cross-platform version is in the works.

See ESPtool on Github and Christopher Rust's accompanying YouTube tutorial.

For NodeMCU or MicroPython, use ESPlorer or NodeMCU Studio to upload the Lua or Python scripts after uploading the initial firmware.