Short video instructions:

• https://www.youtube.com/watch?v=P8Z-ZHwNeNI
• https://www.youtube.com/watch?v=9QZkCQSHnko

The official GitHub repository for ESP8266 with Arduino IDE: https://github.com/esp8266/Arduino. It also provides howtos for different types of installation. I prefer the Board Manager method. Enter

http://arduino.esp8266.com/stable/package_esp8266com_index.json

into Additional Board Manager URLs field. You can add multiple URLs, separating them with commas.