Installing

Boards Manager

This is the suggested installation method for end users.

Prerequisites

Arduino 1.6.8, get it from Arduino website.
Internet connection

Instructions

Start Arduino and open Preferences window.
Enter https://arduino.esp8266.com/stable/package_esp8266com_index.json into Additional Board Manager URLs field. You can add multiple URLs, separating them with commas.
Open Boards Manager from Tools > Board menu and find esp8266 platform.
Select the version you need from a drop-down box.
Click install button.
Don't forget to select your ESP8266 board from Tools > Board menu after installation.

For more information on the Arduino Board Manager, see:

https://www.arduino.cc/en/guide/cores

Using git version

This is the suggested installation method for contributors and library developers.

Prerequisites

Arduino 1.6.8 (or newer, current working version is 1.8.5)
git
Python 2.7 (https://python.org)
terminal, console, or command prompt (depending on your OS)
Internet connection

Instructions - Windows 10

First, make sure you don't already have the ESP8266 library installed using the Board Manager (see above)
Install git for Windows (if not already; see https://git-scm.com/download/win)
Open a command prompt (cmd) and go to Arduino default directory. This is typically the sketchbook directory (usually C:\users\{username}\Documents\Arduino where the environment variable %USERPROFILE% usually contains C:\users\{username})

Clone this repository into hardware/esp8266com/esp8266 directory.

cd %USERPROFILE%\Documents\Arduino\
if not exist hardware mkdir hardware
cd hardware
if not exist esp8266com mkdir esp8266com
cd esp8266com
git clone https://github.com/esp8266/Arduino.git esp8266

You should end up with the following directory structure in

C:\Users\{your username}\Documents\

Arduino
|
--- libraries
--- hardware
    |
    --- esp8266com
        |
        --- esp8266
            |
            --- bootloaders
            --- cores
            --- doc
            --- libraries
            --- package
            --- tests
            --- tools
            --- variants
            --- platform.txt
            --- programmers.txt
            --- README.md
            --- boards.txt
            --- LICENSE

Initialize the submodules

cd %USERPROFILE%\Documents\Arduino\hardware\esp8266com\esp8266
git submodule update --init

If error messages about missing files related to SoftwareSerial are encountered during the build process, it should be because this step was missed and is required.

Download binary tools
```
cd esp8266/tools
python get.py
```
Restart Arduino
If using the Arduino IDE for Visual Studio (https://www.visualmicro.com/), be sure to click Tools - Visual Micro - Rescan Toolchains and Libraries
When later updating your local library, goto the esp8266 directory and do a git pull
```
cd %USERPROFILE%\Documents\Arduino\hardware\esp8266com\esp8266
git status
git pull
```

Note that you could, in theory install in C:\Program Files (x86)\Arduino\hardware however this has security implications, not to mention the directory often gets blown away when re-installing Arduino IDE. It does have the benefit (or drawback, depending on your perspective) - of being available to all users on your PC that use Arduino.

Instructions - Other OS

Open the console and go to Arduino directory. This can be either your sketchbook directory (usually <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

Initialize the submodules
```
cd esp8266
git submodule update --init
```

If error messages about missing files related to SoftwareSerial are encountered during the build process, it should be because this step was missed and is required.

Download binary tools
```
cd esp8266/tools
python get.py
```
Restart Arduino
When later updating your local library, goto the esp8266 directory and do a git pull
```
cd hardware\esp8266com\esp8266
git status
git pull
```