This is the suggested installation method for end users.
http://arduino.esp8266.com/stable/package_esp8266com_index.json into Additional Board Manager URLs field. You can add multiple URLs, separating them with commas.You may optionally use staging boards manager package link:
http://arduino.esp8266.com/staging/package_esp8266com_index.json. This may contain some new features, but at the same time, some things might be broken.
This is the suggested installation method for contributors and library developers.
<Documents>/Arduino), or the directory of Arduino application itself, the choice is up to you.Clone this repository into hardware/esp8266com/esp8266 directory. Alternatively, clone it elsewhere and create a symlink, if your OS supports them.
cd hardware
mkdir esp8266com
cd esp8266com
git clone https://github.com/esp8266/Arduino.git esp8266
You should end up with the following directory structure:
Arduino
|
--- hardware
    |
    --- esp8266com
        |
        --- esp8266
            |
            --- bootloaders
            --- cores
            --- doc
            --- libraries
            --- package
            --- tests
            --- tools
            --- variants
            --- platform.txt
            --- programmers.txt
            --- README.md
            --- boards.txt
            --- LICENSE
Download binary tools
cd esp8266/tools
python get.py
Restart Arduino