Flashing the NodeMCU firmware on the ESP8266 (Linux) - Guide
I wrote also a windows guide, and most of it is relevant here as well, so make sure you go over it before: Windows guide First »
Flashing the NodeMCU firmware on the ESP8266 (Windows) - Guide
EDIT (07-Mar-15): If you own version 12 of the ESP8266 or any version where more GPIO are exposed read here about some additional info for flashing »